20/20 -the expression for normal eyesight (or 6/6 in countries where metric measurements are used). This notation is expressed as a fraction. The numerator (1st number) refers to the distance you were from the test chart, which is usually 20 feet (6 meters). The denominator (2nd number) denotes the distance at which a person with normal eyesight could read the line with the smallest letters that you could correctly read. For example, if your visual acuity is 20/100 that means that the line you correctly read at 20 feet could be read by a person with normal vision at 100 feet. The Snellen chart, which consists of letters, numbers, or symbols, is used to test visual acuity (sharpness of eyesight). A refraction test is used to determine the amount of correction needed for a prescription when treating refractive error such as astigmatism, myopia, or hyperopia. See “Refraction Test”.

Ablation Removal. In vision, ablation refers to the surgical removal of eye tissue to correct a refractive error such as myopia.

AC/A Ratio- accommodative convergence / accommodative (measured in prism diopters/diopters). The convergence response of an individual (amount the eyes turn inward) in relation to the amount of stimulus of accommodation (eye focusing). The normal ratio is 4:1.

Accommodation- (eye focusing) the eye's ability to adjust its focus by the action of the ciliary muscle, which increases the lens focusing power. When this accommodation skill is working properly, the eye can focus and refocus quickly and effortlessly, which is similar to an automatic focus feature on a camera. The ciliary muscles must contract to adjust for near vision, which causes the eye’s crystalline lens, which is flexible, to be squashed. For distant vision, the ciliary muscle must relax and the eye’s crystalline lens is stretched out.  The ability of the eye to accommodate does decrease with age due to the crystalline lens becoming less flexible causing a condition called presbyopia. (See "Presbyopia").

Accommodative Esotropia- (clinical condition) when an individual is focusing on a near object and his or her eyes are turning inward too much. It is caused by either uncorrected hyperopic refractive error and/or a high accommodative convergence/accommodation (AC/A) ratio. The average age of onset is 2 1/2 years. It is most noticeable when the child is tired or sick. This is treated with plus lenses (glasses or contacts) to help straighten the eyes. In some cases, vision therapy and corrective lenses are prescribed. (Please note that Accommodative Esophoria is a condition similar to accommodative esotropia but lesser in extent.)

Accommodative Excess (AE)-  This clinical condition is also called accommodative spasm. It is an over focusing, over stimulation of the focusing action of the crystalline lens causing an inability to relax the focusing system which may result in blurry vision when focusing at distance objects. Other symptoms include holding near work closer than normal, headaches with near work (such as reading or using a computer), eyestrain associated with near work, and possible double vision. Clinical signs include: patient accepts more minus on accommodative rock but blurs with plus lenses, lower NRA than PRA, dynamic retinoscopy findings indication of over accommodation and/or slow relaxation of accommodation, and reduced or erratic distance visual acuity. Treatment includes a low plus lens and/or vision therapy.

Accommodative Fatigue- This clinical condition is also called Ill-Sustained Accommodation. It is the inability of the eye to adequately sustain sufficient focusing over an extended time period. The most common sign or symptom is blurred vision after prolonged near work such as reading and using a computer. In addition, such patients often have asthenopia (eyestrain), general fatigue, headaches and nausea, excess tearing, and an unusual sensitivity to light. Clinical signs include: normal amplitude of accommodation, decreased PRA, and the patient generally fails the +/-2.00 D flipper test. Plus lenses (glasses or contacts) and vision therapy are effective in treating this condition.

Accommodative Insufficiency (AI)-This clinical condition is also called non-presbyopic accommodative insufficiency. It is an under focusing, a lack of focusing ability at a near distance. Symptoms include eyestrain, blurred vision, occasional or constant when doing near work (such as reading or using a computer), occasional unusual sensitivity to light, excess tearing, headaches, and general fatigue. Clinical signs include: patient will have difficulty with a minus lens, low amplitude of accommodation, low PRA and higher NRA. Vision therapy is an effective treatment option.

Accommodative Vergence- a convergence response (to turn the eyes inward) which occurs as a direct result of accommodation (eye focusing). (See "Vergence")

Acetate-Type of plastic often used in eyeglass frames.

Acuity- clearness of eyesight. Depends on the sharpness of images and the sensitivity of nerve elements in the retina. (See "Near Acuity" and "Distance Acuity")

Add- prescription strength of a plus lens which is used for near vision. A plus lens can be added to another lens such as a minus lens for distance vision. (See "Bifocal Glasses" and "Presbyopia")

After-image- the eye's ability to still see an image during eye blinks and even after the viewed object is no longer present. The most common example is seeing light after the flash of a camera.

AK- (astigmatic keratotomy) Procedure in which a surgeon cuts the cornea so that is more spherical when it heals, thus reducing astigmatism.

Albinism- pigmentation is deficient or absent. May occur in skin, hair, and eyes. Ocular albinism is a pigmentation deficiency occurring mainly in the eyes. Individuals with albinism including ocular albinism commonly have decreased visual acuity (20/70 -20/200), strabismus, photophobia, and nystagmus. There is no known treatment. Individuals may benefit from low-vision aids. Treatment options for strabismus and nystagmus does apply to these individuals. For more information about albinism, click here. (See “Strabismus” and “Nystagmus”)

Alignment- proper fusing (uniting) of images to each eye.

Amblyopia- (clinical condition) reduced visual acuity (poorer than 20/20) which is not correctable by glasses or contacts and is not caused by structural or pathological anomalies. This condition is often called “lazy eye” because it is typically the result of disuse. It is usually marked by blurred vision in one eye and favoring one eye over the other. About two percent of the population is affected.

Types of functional (reversible) amblyopia:

• refractive- anisometropia (the two eyes have different refractive powers), or other amblyopiogenic refractive errors (hyperopia, myopia, or astigmatism)
• strabismic- misalignment of the two eyes in which they point in different directions
• form deprivation (may also be referred to as  amblyopia ex anopsia)- caused by conditions that prevent light from entering the eye. These may include congenital ptosis (droopy eyelid), corneal opacity, or cataract.

Treatment options for functional amblyopia are eye patching, prescription lenses and prisms.  

Ametropia- any optical error such as hyperopia, myopia, or astigmatism. Also called refractive error.

AMD or ARMD-(age-related macular degeneration) Disorder characterized by the gradual loss of central vision due to a damaged macula (which is made up of retinal cones necessary for sight).

Amplitude of Accommodation (AA)- a measurement of the eye’s ability to focus clearly on objects at near distances. This eye focusing range for a child is usually about 2-3 inches. For a young adult, it is 4-6 inches. The focus range for a 45-year-old adult is about 20 inches. For an 80-year-old adult, it is 60 inches.

Angle- (angle In glaucoma), "angle" refers to the drainage channel for the aqueous humor in the eye; improper drainage can lead to the high intraocular pressure associated with glaucoma. In narrow-angle glaucoma, the channel is blocked, whereas open-angle glaucoma has other causes, such as the body producing too much aqueous humor.

Aniseikonia- a difference in the size or shape of two visual images when the images should be the same size and/or shape.

Anisometropia- the condition in which the two eyes have different refractive powers.

Anomalous Retinal Correspondence (ARC)- a type of retinal projection, occurring frequently in strabismus, in which the foveae (center of the retina that produces the sharpest eyesight) of the two eyes do not facilitate a common visual direction; the fovea of one eye has the same functional direction with an extrafoveal (non-fovea) area of the other eye.

ANSI Z87.1-The American National Standards Institute's Practice for Occupational and Educational Eye and Face Protection; eyewear that meets this standard is considered safer than eyewear that does not.

Anterior Chamber-Part of the eye behind the cornea and in front of the iris and lens.

Antioxidant-Substance that inhibits oxidation and can guard the body from the damaging effects of free radicals. Molecules with one or more unpaired electrons, free radicals can destroy cells and play a role in many diseases. Antioxidants may help prevent macular degeneration and other serious eye diseases.

Antireflective coating-(AR coating) Thin layer(s) applied to a lens to reduce the amount of reflected light and glare that reaches the eye.

Aperture Rule- a stick-like instrument used in vision therapy to develop convergence and divergence (eye teaming) skills.

Aqueous Humor-Clear fluid in the eye that both provides nutrients and determines intraocular pressure.

Aspheric-Not quite spherical. Aspheric eyeglass lenses are popular among people who have strong prescriptions because they are thin and lightweight, and reduce distortion and eye magnification. Aspheric contact lenses can work as a multifocal, or to correct a single-vision problem like astigmatism.

Asthenopia- eyestrain, symptoms include excessive tearing, itching, burning, visual fatigue, and headache. It can be caused from an uncorrected refractive error, accommodation (eye focusing) disorder, binocularity (eye teaming) disorder, or by extended, intense use of the eyes.

Astigmatism- light rays entering the eye do not all meet at the same point (similar to a frayed string), which results in blurred or distorted vision. An abnormally shaped cornea typically causes this condition. Occasionally astigmatism exists in the lens of the eye. This condition is corrected by a cylindrical (toric) eyeglass or contact lens.

Automated Refractor- also called auto refractor. This method determines the eye's refractive error and the best corrective lenses to be prescribed by using a computerized device that varies its optical power mechanically and prints out the results.

Axis- the alignment of the len’s cylindrical part; used for correcting astigmatism. This measurement is given in degrees. The values are typically from 90 degrees to 180 degrees.

Band Keratopathy- Opacity of the eye's stroma and Bowman's membrane.

Base-Down (BD) Prism- a wedge-shaped lens which is thicker on one edge than the other. The thicker edge (base) is turned down. Prisms bend light (opposite direction from its thicker end) so the base-down prism turns the light upward thus causing the eye to also move up. This prism is used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). Prisms are sometimes added to glasses to help improve eyesight due to a misalignment or visual field loss.

Base-In (BI) Prism- a wedge-shaped lens which is thicker on one edge than the other. The thicker edge (base) is turned inward, closest to the nose. Prisms bend light (opposite direction from its thicker end) so the base-in prism turns the light outward (toward the ear) thus causing the eye to also move outward. This prism is used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). Prisms are sometimes added to glasses to help improve eyesight due to a misalignment or visual field loss.

Base-Out (BO) Prism- a wedge-shaped lens which is thicker on one edge than the other. The thicker edge (base) is turned outward, closest to the ear. Prisms bend light (opposite direction from its thicker end) so the base-out prism turns the light inward (toward the nose) thus causing the eye to also move inward. This prism is used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). Prisms are sometimes added to glasses to help improve eyesight due to a misalignment or visual field loss.

Base-Up (BU) Prism- a wedge-shaped lens which is thicker on one edge than the other. The thicker edge (base) is turned up. Prisms bend light (opposite direction from its thicker end (base)) so the base-up prism turns the light downward thus causing the eye to also move down. This prism is used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). Prisms are sometimes added to glasses to help improve eyesight due to a misalignment or visual field loss.

Bifocal Glasses- used to correct vision at two distances, composed of two ophthalmic lenses such as a plus lens for near vision and a minus lens for distance vision.

Binocular Vision- the simultaneous use of the two eyes.

Binocularity-the ability to use both eyes as a team and to be able to fuse (unite) two visual images into one, three-dimensional image (See “Convergence” and “Divergence”).

Blepharitis-Condition characterized by crusting around the eyes upon awakening, itching, burning, tearing, swollen eyelids and mucus.

Blurred Vision- lack of visual clarity or acuity.

Botulinum Toxin Type A (Oculinum, Botox^®)- an injection of this poison has been used as an alternative to conventional surgery in selected strabismic patients. It causes a temporary paralysis of an extraocular muscle that leads to a change in eye position. This change has been reported to result in long-lasting and permanent alteration in eye alignment. Although one injection is often sufficient to produce positive results, one-third to one-half of patients may require additional injections. This technique has been most successful when used in adults with small-angle misalignments. It is not commonly used in children. This treatment is also used in patients who have blepharospam (an uncontrollable eye lid spasm).

Bowman's Membrane- Corneal layer between the epithelium and the stroma.

Break Point- the point at which a person can no longer fuse (unite) two images into one. A blur point will occur before the this point.

Brewster Stereoscope– an instrument used in orthoptics/vision therapy to improve eye teaming skills and near focusing skills. It consists of two parallel viewing tubes with a +5.00 D lens. The distance from the target can be adjusted as well as the pupillary distance. The Bernell-O-Scope and Keystone Ophthalmic Telebinocular are designed essentially the same.

Bridge– The part of eyeglasses that extends across the nose.

Cable temple- Style of eyeglasses that wraps around the ear, to keep them well-fastened.

Cataract- a condition of the crystalline lens, in which the normally clear lens becomes clouded or yellowed, causing blurred or foggy vision. Cataracts may be caused by aging, eye injuries, disease, heredity, or birth defects. Surgery is a treatment option. The affected lens is removed and is replaced with a substitute (implant) lens or with a special type of contact lens. Generally the success rate of cataract surgery is over 90%, if the eye is otherwise healthy.

Central island- Refractive surgery complication in which the laser leaves an "island" of corneal tissue in the concave ablation zone. Symptoms include double vision and distortion.

Choroid- Layer of blood vessels and pigments (usually brown or blue) that lies beneath the sclera (the white of the eye). Often mistakenly called the iris.

Ciliary Body - a structure directly behind the iris of the eye and contains the ciliary muscle.

Ciliary Muscle- a band of muscle and fibers that are attached to the lens that controls the shape of the lens and allows the lens to accommodate (change focus).

CMV retinitis- (cytomegalovirus retinitis) Serious eye infection usually found in those with immune problems, such as AIDS patients; symptoms include floaters, blind spots, blurry vision and vision loss.

Collagen- Fibrous protein in bones and connective tissue, it is also present in the eye. One type of vision correction surgery heats collagen around the edges of the cornea (which lets light into the eye). This procedure reshapes the cornea, helping it focus light right onto the retina, for clearer vision.

Color Perception Test- a test that measures the ability to identify and distinguish colors.

Color Vision Deficiency- also known as Colorblindness. It is the absence of or defect in the perception of colors. Color vision is based on perception of red, green, and blue. If there is a defect in the perception of one of these colors, a color will be perceived as if it were composed only of the other two colors. Based on the color or colors for which there is defective perception, a person may suffer from red, green, or blue blindness. Color blindness in which all colors are perceived as gray is termed monochromasia. For people with the common, inherited, types of color deficiency there is no cure.

Comitant Strabismus- a condition in which the magnitude of deviation remains essentially the same in all positions of gaze and with either eye fixating.

Computer Vision Syndrome (CVS)- the complex of eye and vision problems related to near work that are experienced during or related to computer use. Its symptoms include eyestrain, dry or burning eyes, blurred vision, headaches, double vision, distorted color vision, and neck and backaches. The condition is caused by various internal and external factors. Treatment options are prescription glasses, frequent breaks from near focusing and eye lubrication.

Conductive Keratoplasty-(CK) Procedure wherein a surgeon uses radio waves to heat collagen in the cornea's periphery to shrink it and reduce hyperopia.

Cone – a receptor cell which is sensitive to light and is located in the retina of the eye. It is responsible for color vision.

Conjunctiva- Mucous membrane that lines the visible part of the eye and the inner surface of the eyelid.

Conjunctivitis- an inflammation of the conjunctiva, the transparent layer covering the inner eyelid and the white portion (sclera) of the eyeball. Conjunctivitis can be caused by a virus, bacteria, or fungus (infectious conjunctivitis, or "pink eye", may be contagious); by allergies to pollen, fabrics, animals, or cosmetics (allergic conjunctivitis); or by air pollution or noxious fumes such as swimming pool chorine (chemical conjunctivitis). Symptoms include red or watery eyes, blurred vision, inflamed inner eyelids, scratchiness in the eyes, or (with infectious conjunctivitis) a puss like or watery discharge and matted eyelids. Conjunctivitis is usually treated with antibiotic eye drops and/or ointment.

Convergence- the ability to use both eyes as a team and to be able to turn the eyes inward to maintain single vision up close.

Convergence Excess (CE)- a clinical condition in which the eyes have a tendency to turn excessively inward when viewing an object at a near distance. Symptoms may include visual fatigue while reading or using a computer, occasional blurred or double vision, and inability to comprehend or concentrate while reading. Clinical signs include: greater esophoria at near than distance, high AC/A ratio, and a high lag of accommodation. Can be improved with orthoptic therapy and/or glasses. (See "Esophoria")

Convergence Insufficiency (CI)- (clinical condition) the inability of the eyes to turn inward and/or sustain an inward turn. Symptoms include eye strain with reading and using a computer, headaches, loss of comprehension, difficulty concentrating, blurred or double vision, and eye fatigue. Clinical signs include: near point of convergence of greater than 4 inches (10 cm), greater exophoria at near than at distance, and low AC/A ratio. Orthoptic therapy is an effective treatment option.

Cornea- the transparent, blood-free tissue covering the central front of the eye (over the pupil, iris, and aqueous humor) that initially refracts or bends light rays as light enters the eye. Contact lenses are fitted over the cornea.

Corneal Abrasion-Tearing or puncture of the cornea. Usually causes pain, tearing, light sensitivity, and a feeling that something is in the eye.

Corneal Implants-Devices (such as rings or contacts) placed in the eye, usually to correct vision.

Corneal Ring-Type of vision correction surgery where a doctor inserts a tiny plastic ring into the cornea (which lets light into the eye). This ring reshapes the cornea, helping it to focus light better onto the retina so you can see better. The ring can be adjusted and even removed if desired.

Corneal Topography-Process of using a camera/computer system to map the cornea for refractive surgery, contact lens fitting and corneal disease management.

Corneal Ulcer-Wound in the surface of the eye caused by injury, dryness due to lack of tear production, or infection.

Cover Test- a test of eyeball alignment in which each eye is covered with an occluder (eye cover) and then uncovered to observe eye movements.

Cystoid Macular Edema- (CME) Swelling of the eye's macula, caused by an excessive amount of fluid.

Crystalline Lens- transparent disc located behind the iris which changes shape to focus on objects at different distances from the eye.

Cycloplegic Refraction- one method available to eye doctors to determine the eye's refractive error and the best corrective lenses to be prescribed if needed. The eye is dilated with the muscles of accommodation (eye focusing muscles) being temporarily paralyzed with specialized eye drops or spray (Atropine, Homatropine, Cyclogyl, or Mydriacyl). This is a good method for non-responsive or non-communicative patients such as young children. The technique of retinoscopy is used with this method. (See “Retinoscopy”)

Cylinder Lens- an ophthalmic lens that has at least one non-spherical surface. Used to correct astigmatism. The values are typically from -0.75 to -1.25. The cylinder measurement is given with a "-" sign. (Please note that the sign for myopia (nearsightedness) is also "-".)

Dacryostenosis- Blocked tear duct, which is characterized by a lot of tearing.

Depth Perception- the ability to judge relative distances of objects. (See "Stereopsis")

Depth Perception Test- a test to measure the ability of the vision system to discern the relative distances of various objects. (Also called a “Stereopsis Test”)

Descemet's Membrane- Corneal layer between the stroma and the endothelium.  

Developmental Disorder- when a delay in an individual’s normal development has occurred.

Diabetic Retinopathy- Leaking of retinal blood vessels in advanced or long-term diabetes, affecting the macula or retina. Vision can be seriously distorted or blurred.

Diopter (D)- a measurement of the refractive (light bending) power of a lens or a prism (pd). The strength of prescription glasses and contacts are measured in these units. For example a lens that is 0.50 diopter (D) is very weak, where as a lens that is 10.0 diopter (D) is very strong.

Diplopia- a single object is perceived as two rather than one; double vision.

Direct Occlusion- covering the non-amblyopic eye. (See "Inverse Occlusion" and "Occlusion")

Directionality/Laterality- directionality relates to the awareness of the relationship of one object in space to another / laterality relates to the internal awareness of the two sides of the body. Directionality/Laterality can also be called spatial relations.

Directionality/LateralityDisorder- a condition in which an individual has poor development of left/right awareness. Symptoms of this disorder include confusion of right and left direction and letters and/or numbers reversals. Vision therapy is a helpful treatment option.

Distance Acuity-the eye's ability to distinguish an object's shape and details at a far distance such as 20 feet (6 meters).

Divergence- the ability to use both eyes as a team and be able to turn the eyes out toward a far object.

Divergence Excess (DE)- (clinical condition) the eye's tendency to drift out relative to the direction of a distant object being viewed. Symptoms include: double vision at distance, headaches, eyestrain, nausea, dizziness, and blurred vision. Clinical signs include: exophoria greater at distance than near, high AC/A ratio, and reduced positive fusional vergence at distance. Can be improved with vision therapy.

Divergence Insufficiency (DI)- (clinical condition) the eye's tendency to turn more inward than necessary when viewing a distant object. Symptoms include: double vision, headaches, eyestrain, nausea, dizziness, and blurred vision. Clinical signs: esophoria greater at distance than near, low AC/A ratio, and reduced negative fusional vergence at distance. Treated with corrective lenses and vision therapy.

Dominant Eye-the eye that "leads" it partner during eye movements. Humans also have dominant hand, foot, eye, and side of the brain (not necessarily all on the same side).

Druse- Small yellow or white deposit in the eye. Drusen are sometimes signs of macular degeneration.

Dry Eye- Lack of sufficient lubrication and moisture in the eye. Most dry eye complaints are temporary and easily relieved; dry eye syndrome is chronic and needs more advanced treatment by an eyecare practitioner.

Duction Test- a test of the eye's ability to turn inward or outward while maintaining single, binocular vision with the gradual introduction of progressively stronger base-in or base-out prisms.

Dyslexia– a specific language-based disorder. The individual has difficulty with letter or word recognition, spelling, reading, writing, and sometimes naming pictures of objects. Dyslexia varies in degree from mild to very sever. It is caused by an inability of the brain's language centers to decode print or phonetically make the connection between the word's written symbols and their appropriate sounds. Dyslexia is not caused by a vision disorder. Children often are of normal or above normal intelligence. Dyslexia cannot be cured and will never be outgrown. Appropriate teaching methods can be taught to help those with dyslexia overcome their weakness.

Eccentric Fixation- the deviating eye does not use the central foveal (center of the retina that produces the sharpest eyesight) area for fixation. Commonly, individuals with amblyopia and some individuals with strabismus will have this visual adaptation. In esotropia, the eccentrically located retinal point used for fixation is usually in the nasal retina. In exotropia, the eccentrically located retinal point used for fixation is usually in the temporal retina. Vision therapy is a treatment option for those with amblyopia and/or strabismus. It is not a treatment option for an individual with a fovea that has been destroyed.

Emmetropia- normal vision, no correction needed. (See diagram of the eye.) 

Esophoria (Eso)- (clinical condition) a tendency of the eyes to want to turn more inward than necessary when an individual is viewing an object at near or at distance, which may cause the individual to experience eyestrain and other symptoms. Symptoms of basic esophoria include: eyestrain, headaches, blurred or double vision, apparent movement of print, and difficulty concentrating on and comprehending reading material. Clinical signs of basic esophoria include: AC/A ratio is normal, equal esophoria at distance and near, and normal near point of convergence. Sometimes esophoria is caused by a refractive error such as hyperopia (farsightedness), and glasses or contacts can correct the problem alone. However, sometimes vision therapy is needed to to help re-train the eyes to function more appropriately.

Endothelium- The cornea's inner layer of cells.

Epithelium- The cornea's outer layer of cells.

Esotropia (ET)- (clinical condition) a condition in which an eye is turned either constantly or intermittently inward toward the nose. Esotropia is a type of strabismus. It is caused by a reduction in visual acuity, reduced visual function, high refractive error, traumatic brain injury, oculomotor nerve lesion, or eye muscle injury. Treatment options may include one or more of the following: glasses or contacts, bi-focal lenses, prisms, vision therapy, surgery, or Botulinum Toxin Type A (Oculinum, Botox^®) injections. In some cases, esotropia is caused by a refractive error such as hyperopia (farsightedness), and glasses or contacts alone may allow the eyes to straighten. Vision therapy is most appropriate when there are small degrees of misalignment. Surgery, to re-position or shorten the eye muscles, may be required for high degrees of misalignment. If surgery is required, a combination of surgery and vision therapy often yields the best results.

Exophoria (Exo)- (clinical condition) a tendency of the eyes to want to turn more outward than necessary when an individual is viewing an object at near or at distance, which may cause the individual to experience eyestrain and other symptoms. Symptoms of basic exophoria include: eyestrain, headaches, blurred or double vision, apparent movement of print, and difficulty concentrating on and comprehending reading material. Clinical signs of basic exophoria include: normal AC/A ratio, equal exophoria at distance and near, and decreased near point of convergence. Vision therapy is an effective treatment option.

Exotropia (XT)- (clinical condition) a condition in which an eye is either constantly or intermittently turned outward toward the ear. Exotropia is a type of strabismus. It may also be called divergent strabismus, wandering eye, or wall eye(s). It is caused by a reduction in visual acuity, reduced visual function, high refractive error, traumatic brain injury, oculomotor nerve lesion, or eye muscle injury. Treatment options may include one or more of the following: glasses or contacts, bi-focal lenses, prisms, vision therapy, surgery, or Botulinum Toxin Type A (Oculinum, Botox^®) injections. Vision therapy is most appropriate when there are small degrees of misalignment. If surgery is required, a combination of surgery and vision therapy often yields the best results. For more information, please click here. To see the American Optometric Association's guidelines for vision therapy, please click here.(See “Strabismus”)

Extraocular Muscles- the muscles attached to the outside of the eyeball which control eye movement. Each eye has six muscles (lateral rectus, medial rectus, superior oblique, inferior oblique, superior rectus, and inferior rectus) that are coordinated by the brain.

Eye Hand Coordination- the ability of our eyes to guide our hands, also called visual motor integration.

Eye Trac- (equipment) an electronic testing and recording system of eye movements as in reading.

Eye Tracking- the ability of the eyes to smoothly and effortlessly follow a moving target.

Eyecare Practitioner- Optometrists (O.D.s) and ophthalmologists (M.D.s) both practice eyecare, but in different ways: O.D.s (Doctors of Optometry) prescribe glasses, prescribe and fit contact lenses and treat limited eye conditions and diseases. M.D.s are medical doctors who examine eyes, prescribe glasses and contacts, treat all eye diseases and conditions, and perform surgery.

Facility of Accommodation- a measure of the ease and speed of the eye(s) to change focus.

Farsighted- Also called hyperopia. To farsighted people, near objects are blurry, but far objects are in focus.

Figure-Ground- the ability to recognize distinct shapes from their background, such as objects in a picture, or letters on a chalkboard.

Fine Motor Skills- the ability to coordinate hand and finger movements.

Fixation- the ability to direct and maintain steady visual attention on a target. Fixations are a form of pursuits.

Fixation Disparity (FD)- over-convergence or under-convergence, or vertical misalignment of the eyes under binocular (both eyes) viewing conditions small enough in magnitude so that fusion is present.

Flap and Zap- Slang for LASIK.

Floaters- also known as spots, are usually clouded or semi-opaque specks or particles within the eye that are seen in the field of vision. The eyes are filled with fluid which maintains the shape of the eye, supplies it with nutrition and aids in the focusing of light. Often, particles of protein or other natural materials are left floating or suspended in this fluid when the eye is formed before birth. If the particles are large or close together, they cast shadows which make them visible. This is particularly true when nearsightedness occurs or becomes more severe. In most cases this is normal but floaters can also be caused by certain injuries, eye disease or deterioration of eye fluid or its surrounding structures.

Form Constancy- the ability to recognize two objects that have the same shape but different size or position. This ability is needed to tell the difference between "b" and "d", "p" and "q", "m" and "w".

Fovea- center of the retina that can produce the sharpest eyesight and contains the most cones.

Fusional Vergence-  a convergence response which serves to maintain (fusion) the union of images from each eye into a single image. The eyes will turn with a slow smooth tonic movement or a fast jumping movement called phasic.

Fusional Vergence Dysfunction

Fusion- the union of images from each eye into a single image.  There are three degrees of fusion. 1^st degree fusion is the superimposition of two dissimilar targets. 2^nd degree fusion is flat fusion with a two-dimensional target. 3^rd degree fusion is depth perception (stereopsis) with a three-dimensional target.

Fusion Test- determines the eyes ability to unite the images from each eye into a single image.

Glaucoma- Disease characterized by excessive fluid (aqueous humor) in the eye, high intraocular pressure and vision impairment. Blindness can result.

Graves' Ophthalmopathy- Thyroid-related, autoimmune eye disorder usually associated with Graves' disease; symptoms include eyelid retraction, bulging eyes, light sensitivity, discomfort, double vision and vision loss.

Heterophoria- tendency of the eyes to deviate from their normal position for visual alignment. This condition may be observed when one eye is covered.

Heterotropia- the eyes are abnormally turned.

Higher-Order Aberration- Irregularity of the eye other than a refractive error (myopia, hyperopia or astigmatism). Higher-order aberrations sometimes affect your vision (such as decreasing contrast sensitivity), and sometimes do not.

High Index- Type of lens with a higher index of refraction, meaning that light travels faster through the lens to reach the eye than with traditional glass or plastic. It is denser, so the same amount of visual correction occurs with less material (whether glass or plastic) — so the lens can be thinner.

Hyperopia- farsightedness, an individual will have difficulty seeing clearly up close. Light entering the eye focuses behind the retina when the eye is at rest and is corrected with a plus lens. Vision therapy is not prescribed for hyperopia. Children, up to about the age of 8 years, are often farsighted.

Hyperphoria- a condition in which one eye has a tendency to point higher than the other eye, causing eyestrain. Sometimes improved by prisms in glasses.

Hypertropia- strabismus, one eye turned in an upward direction.

Hypophoria- a condition in which one eye has a tendency to point lower than the other eye. This condition may be observed when one eye is covered.

Hypotropia- strabismus, one eye turned in a downward direction.

Hysterical Amblyopia- a non specific visual loss with an unknown cause. Upon examination the doctor is unable to find corroborating objective evidence of this abnormality. The most common symptom is an isolated visual acuity impairment, followed by combined visual acuity impairment and visual field constriction, and whereas an isolated visual field constriction occurred most infrequently. This vision loss may be due to anxiety or emotional repression. (See "Streff Syndrome")

Ill-Sustained Accommodation- this clinical condition is also called Accommodative Fatigue. It is the inability of the eye to adequately sustain sufficient focusing over an extended time period. The most common sign or symptom is blurred vision after prolonged near work such as reading and using a computer. In addition, such patients often have asthenopia (eyestrain). Clinical signs include: normal amplitude of accommodation, decreased PRA, and the patient generally fails the +/-2.00 D flipper test. Plus lenses (glasses or contacts) and vision therapy are effective in treating this condition.

IncomitantStrabismus - a condition also known as Noncomitant Strabismus. It occurs when the magnitude of deviation is not the same in the different positions of gaze or with either eye fixating.  There is an abnormal restriction to movement or an over-action of one or more of the extraocular muscles. Generally, the magnitude must change by at least 5 PD to be incomitant (nonconcomitant).

Intraocular Lens- (IOL) Artificial lens that a cataract surgeon places in a patient's eye after removing the eye's natural lens. Like a contact lens, it has a built-in refractive power tailored specifically to the patient's visual condition.

Inverse Occlusion- covering the amblyopic eye. (See "Direct Occlusion" and "Occlusion")

Intraocular Pressure- (IOP) Eye pressure, as determined by the amount of aqueous humor filling it. High IOP (ocular hypertension) can be a sign of glaucoma.

Iris- the colored part of the eye located between the lens and cornea; it regulates the entrance of light.

Iritis- Inflammation of the iris.

Keratectomy- Surgical removal of part of the cornea.

Keratitis- Inflammation of the cornea.

Keratoconus- Condition in which the cornea develops a cone-shaped bulge that can result in major blurring and distortion.

Keratoplasty- Any of several types of corneal surgery, such as shrinking the collagen to reduce farsightedness or transplanting a new cornea to treat keratoconus.

keratotomy- Incision of the cornea.

Kinesthesia- the sensation of bodily position, presence, or movement resulting chiefly from stimulation of sensory nerve ending in muscles, tendons, and joints.

Lacrimal Plug- Device to block the lacrimal punctum (an opening at the end of a tear duct), to keep the eye moist.

Lag of Accommodation- a measure of the eye's ability to focus accurately on a given target. The dioptric difference between the eye's focusing response and the stimulus to focus.

Laser Photocoagulation- Procedure in which a surgeon uses a laser to coagulate tissue, usually to seal leaking blood vessels and destroy new ones in diseases like macular degeneration and diabetic retinopathy.

LASEK- (Laser Epithelial Keratomileusis) Procedure that is similar to LASIK, except that the surgeon cuts a flap in the epithelium only, instead of through the epithelium and part of the stroma. LASEK is used mostly for people with thin or flat corneas who are poor candidates for LASIK, which requires more corneal tissue for success.

Laser Thermal Keratoplasty- (LTK) Also called Laser Thermokeratoplasty. Surgery to correct mild farsightedness in people over 40; the doctor uses a holmium laser to heat the cornea and shrink its collagen.

LASIK- (Laser-Assisted In Situ Keratomileusis) Surgical procedure in which a tiny flap is cut in the top of the cornea, underlying corneal tissue is removed with an excimer laser, and the flap is put back in place. LASIK corrects myopia and hyperopia.

Lateral Rectus- muscle Muscle that moves the eye away from the nose.

Latent Hyperopia- hyperopia (farsightedness) is compensated by accommodation and the tonicity (tension) of the ciliary muscle; identified by cycloplegic refraction. In mild cases of hyperopia (farsightedness), the eyes are able to compensate without corrective lenses; otherwise a plus lens (glasses or contacts) is prescribed. Vision therapy is not prescribed. (See "Hyperopia")

Learning Disability (LD)- a disorder that affects people's ability to either interpret what they see and hear or to link information from different parts of the brain. Learning disabilities can be divided into five broad categories: speech and language disorders, reading disorder, arithmetic disorder, writing disorder, and attention disorders. The term learning disability does not include children who have learning problems that are primarily the result of visual, hearing, or motor disorders.

Lensometer-also called Verometer, is a device used to measure the refractive power of eyeglasses and contact lenses.

Limbus- Boundary area connecting the cornea and sclera; the three form the eye's outermost layer.

Lipid- A fatlike substance that can collect on contact lenses, making them uncomfortable.

Low Vision- Also called partial sight. Sight that cannot be satisfactorily corrected with glasses, contacts, or surgery. Low vision usually results from an eye disease such as glaucoma or macular degeneration.

Lutein- An antioxidant that is found throughout the body, but is concentrated in the macula. Lutein is believed to help protect the eyes from free radical damage caused by the sun's harmful rays.

Macula- the most sensitive part of the retina that is about the size of a pinhead and is where our most detailed vision occurs.

Maculopathy- Disease of the macula, such as age-related macular degeneration.

Macular Degeneration –a deterioration of the central portion of the retina known as the macula. Click here for information.

Malingering- a voluntary or intentional reduction in visual acuity or other examination data.

Medial Rectus- muscle Muscle that moves the eye toward the nose.

Microcornea- Abnormally small cornea.

Microkeratome- Small instrument that surgeons use to cut the cornea.

Microstrabimus– also called microtropia, monofixation syndrome, and small angle strabismus. A small angle deviation (inward or outward, commonly inward) that is less than five degrees with some amount of stereopsis (depth perception) and anomalous retinal correspondence (ARC). Possible mild amblyopia, eccentric fixation, and/or anisometropia may also be present. It frequently results from the treatment of a larger-angle deviation (esotropia or exotropia) by optical correction, vision therapy, pharmacological agents, and/or extraocular muscle surgery. Treatment for microstrabimus consists mostly of correcting significant refractive errors and any coexisting amblyopia. The use of vision therapy and prisms to establish bifoveal fusion has been successful in selected cases.

Migraine- (ocular) Visual phenomena that may accompany a migraine headache or that may occur without any headache. They include light flashes, spots, wavy lines, flickers, zig-zagging lights, semi-circular or crescent-shaped visual defects and distortions of shapes.

Minus (-) Lens- concave lens, stimulates focusing and diverges light. The lens is thinner in the center than the edges. It is used in glasses or contact lenses for people who are nearsighted (myopia).

Monocular Vision- only one eye having useful vision.

Myasthenia Gravis- Autoimmune disease sometimes related to thyroid conditions. Signs are droopy eyelids or double vision that worsen toward the end of the day.

Myopia- nearsightedness, an individual will have difficulty seeing clearly at distance. Light entering the eye focuses in front of the retina when the eye is at rest and is corrected with a minus lens. A condition known as high myopia occurs when myopia is greater than 6 diopters. Typically, vision therapy is not prescribed for myopia.

Near Acuity-the eye's ability to distinguish an object's shape and details at a near distance such as 16 inches (40 cm).

Nearsighted- Also called myopia. Condition in which visual images come to a focus in front of the retina, resulting in defective vision of distant objects.

Near Point of Convergence (NPC)- the closest point at which the two eyes can maintain a single united image.

Near Point of Convergence Test- measures the patient’s ability to point the eyes at an approaching object and to keep them fixed on the object as it reaches the patient’s nose.  Normal range is 0 to 4 inches away from the nose.

Negative Relative Accommodation (NRA)-a measure of the maximum ability to relax accommodation while maintaining clear, single binocular vision.

Neovascularization- Abnormal growth of new blood vessels, such as in an excessive amount, or in tissue that normally does not contain them.

Nevus- Also spelled naevus. Brown pigmentation resembling freckle or mole on white of the eye. More often found in dark-skinned people. Sometimes malignant.

Nickel- Metallic element used mainly in alloys. Many eyeglass frames are made of nickel alloy, so people who are allergic should choose a hypoallergenic substitute, such as titanium.

Normal Retinal Correspondence (NRC)- the foveas of the two eyes are corresponding neural points in the visual cortex and binocular vision can occur.

Nystagmus- rhythmic oscillations or tremors of the eyes which occur independent of the normal eye movements. Generally nystagmus is not curable, but it is manageable. Treatments include prescription glasses or contact lenses, prisms, and vision therapy.

Occlusion- to block out light. An eye can be completely or partially blocked. This procedure is used to promote the use of one eye or both eyes. This therapy procedure may be used for people with amblyopia, strabismus, or closed head trauma. An eye patch, black contact, or another device may be used to block out light from an eye. (See "Direct Occlusion" and "Inverse Occlusion")  

Ocular Motility- pertaining to binocular alignment and eye muscle movement. (See "Binocularity", "Strabismus")

Ocular Motor (OM)- general eye movement ability, which include pursuits (to visually track and/or follow moving objects) and saccades (to direct and coordinate eye movement as the eye quickly and voluntarily shift from one target to another).

Ocular Motor Dysfunction- poor eye movement skills. Vision therapy is an effective treatment option. To see the American Optometric Association's guidelines for vision therapy, please click here.  (See “Pursuits Dysfunction” and “Saccades Dysfunction”)

Ocular Hypertension- Condition in which the intraocular pressure of the eye is elevated above normal and which may lead to glaucoma.

Oculomotor Skills-the ability to quickly and accurately move our eyes. These are sensory motor skills that allow us to move our eyes so we can fixate on objects (fixation), move our eyes smoothly from point to point as in reading (saccades), and to track a moving object (pursuits). (See "Fixation", “Pursuits” and “Saccades”)

Oculus Dexter (OD)- right eye.

Oculus Sinister (OS)- left eye.

Oculus Uterque (OU)- both eyes.

Ophthalmologist- a physician (doctor of medicine (M.D.) or doctor of osteopathy (D.O.) who specializes in the comprehensive care of the eyes and visual system in the prevention of eye disease and injury. The ophthalmologist has completed four or more years of college premedical education, four or more years of medical school, one year of internship, and three or more years of specialized medical and surgical training and experience in eye care. The ophthalmologist is a physician who is qualified by lengthy medical education, training and experience to diagnose, treat and manage all eye and visual system problems, and is licensed by a state regulatory board to practice medicine and surgery. The ophthalmologist is the medically trained specialist who can deliver total eye care: primary, secondary and tertiary care services (i.e., vision services, contact lenses, eye examinations, medical eye care and surgical eye care), and diagnose general diseases of the body. An ophthalmologist is not trained to provide vision therapy.

Ophthalmoscope- a device used to illuminate the inside of the eye and enlarge the image for examining the retina, optic nerve entrance, arteries, and veins.

Optic Nerve- is a bundle of nerve fiber that connects each eye to the brain and transmits images from the retina to the brain. (See diagram of the eye)

Optic Nerve Head- Also called optic disk. Circular area where the optic nerve enters the retina, and the location of the eye's blind spot.

Optician- is a professional in the field of designing, finishing, fitting and dispensing of eyeglasses and contact lenses, based on an eye doctor's prescription. The optician may also dispense colored and specialty lenses for particular needs as well as low-vision aids and artificial eyes.

Optometrist- a health care professional who is state licensed to provide eye examinations, prescribe glasses, fit contact lenses and to treat limited eye conditions. The optometrist is not a medical doctor.

Organic Amblyopia- gradual or sudden loss of central vision (partial loss) affecting visual acuity with no treatment options.

Types of organic (irreversible) amblyopia:

• nutritional amblyopia- vision loss caused by low levels of vitamin B12 due to poor nutrition and poor absorption associated with drinking alcohol.
• tobacco-alcohol amblyopia- clinical evidence exists that a nutritional deficiency is the underlying cause of this vision loss; however, many still believe that the toxic effects of alcohol and/or tobacco are contributing factors.
• toxic amblyopia- caused by exposure to toxins such as ethambutol, methyl alcohol (moonshine), ethylene glycol (antifreeze), cyanide, lead, and carbon monoxide.

Orthophoria (ortho)- the absence of either esophoria or exophoria. The eyes do not have a tendency to want to turn more inward than necessary or want to turn more outward than necessary when pointed on an object.

Orthoptics- the science of correcting defects in binocular vision. The technique of eye exercises to correct strabismus (esotropia or exotropia), convergence insufficiency (exophoria), or convergence excess (esophoria), amblyopia, and ocular motility disorders. Orthoptics was pioneered by French ophthalmologist Javal in the mid to late 1800’s. Today ophthalmologists use specialty-trained healthcare professionals called orthoptists to evaluate patients and treat them with orthoptics. In America, the non-surgical technique of orthoptics is less commonly used by ophthalmologists compared to other countries. Orthoptics is a limited form of optometric vision therapy.

Overconvergence- Condition in which the eyes come too far inward when focusing on a near object, resulting in blurring.

Papilla- Small bump where the optic nerve exits the eye.

Paresis- a paralysis that when occurring in ocular muscles causes double vision when looking in some directions.

Pars Plana- Posterior part of the eye's ciliary body.

Penalization- to prevent sight out of the good eye and force the weaker, amblyopic eye, to function. A filter, eye patch, or eye drops such as atropin or miotics are used on the good eye.

Perimetry- the measurement of a visual field function (the total area that can be seen while looking straight ahead) using targets of different sizes and brightness (light levels). The visual field is measured in degrees. In a normal eye the peripheral field of vision is about 180 degrees. An instrument called a perimeter is used for mapping all areas of a person's eyesight, including peripheral (side) vision. Visual field testing can help detect certain patterns of visual loss, indicating specific types of eye diseases or vision conditions. It is the single best test for diagnosing glaucoma.

Peripheral Vision-the ability to see or be aware of what is surrounding us, our side vision. (See "Visual Field".)

Phasic- fast, jump movement. (See "Fusional Vergence")

Photoablation- Procedure in which a surgeon uses ultraviolet radiation to remove tissue.

Photochromic- Able to change lens color or darkness/density depending upon the degree of exposure to light.

Photokeratitis- "Sunburn" of the cornea; symptoms include discomfort, blurred vision, and light sensitivity. The temporary vision loss that can result is called "snow blindness."

Photophobia- unusual sensitivity to light.

Photorefractive Keratectomy- (PRK) Surgical procedure in which an excimer laser is used to remove corneal tissue to correct vision problems.

Pingueculum- Yellowish, thickened lesion on the conjunctiva. May be caused by irritation from sun, dust and wind.

Physiological Diplopia- a normal diplopia (double vision) that occurs when an individual is not pointing his/her eyes on a certain object.

Plano Lens- a lens that has no prescription. No variance between the curvature of the front and back lens surfaces. It is a flat lens.

Pleoptics- a method of eye exercises created to stimulate and train an amblyopic eye. The goal is to have eyesight which is produced by the fovea. (See "Eccentric Fixation")

Plus (+) Lens- convex lens (thicker in the middle) relaxes focusing and converges light. It is typically used in glasses or contact lenses for people who are farsighted (hyperopic). Although it may also be prescribed for other visual conditions as well.

Polaroid Lens- a lens used in sunglasses and sometimes 3D glasses which consists of two glass or plastic surfaces with a plastic lamination between the two surfaces, and designed to reduce reflected glare. In optometric vision therapy, these lens are used with 3D pictures such as vectograms and stereograms, which are also polarized.

Polycarbonate- Lens material that is very impact-resistant, thinner than plastic, and is used for spectacle lenses.

Polymethyl Methacrylate-(PMMA) Old-fashioned hard contacts were made of PMMA, which is not permeable; today's rigid lenses contain other polymers that allow oxygen to reach your eye.

Positive Relative Accommodation (PRA)- a measure of the maximum ability to stimulate accommodation while maintaining clear, single binocular vision.

Posterior Chamber- Part of the eye behind the iris and in front of the lens.

Presbyopia- sometimes called the fourth refractive error, is not truly a refractive error. It is the natural process of the eye losing the ability to accommodate or change the shape of the natural crystalline lens inside the eye to see comfortably at near. This vision defect occurs with the advancement of age; the onset usually occurs between the ages of 40 to 45. Unlike the rest of the body, which stops growing by the age of twenty, the lens of the eye continues to grow throughout life. As the lens ages and grows, it becomes harder in consistency, loses its elasticity, and therefore is resistant to changes in shape. The result is a gradual reduction in accommodation (near eye focus), and a more dependence on reading glasses. A plus lens or multi-focal lens (such as a bifocal lens) is prescribed in the form of glasses or contact lenses. For more information, please click here. Vision therapy is not a treatment option.

Prism- a wedge-shaped lens which is thicker on one edge than the other. This plastic or glass lens bends light (opposite direction from its thicker end). Prisms can be used to measure an eye misalignment and/or treat a binocular dysfunction (eye teaming problem). A prism is sometimes added to glasses to help improve eyesight due to an eye misalignment or visual field loss. (See "Base-Down Prism", “Base-In Prism”, “Base-Out Prism”, "Base-Up Prism", "Yoked Prism")

Prismatic Effect By Lens- when light goes through a wedge shaped lens which is called a prism, it bends. Light is also bent when it does not go through the center of a lens. This is an undesirable effect that can occur in glasses. It commonly occurs when the pupillary distance (PD) is not measured or made correctly.

Progressive Lenses- (also, progressive addition lenses or PALs) Multifocal lenses whose corrective powers change progressively throughout the lens. A wearer looks through one portion of the lens for distance vision, another for intermediate vision, and a third portion for reading or close work. Each area is blended invisibly into the next, without the lines that traditional bifocals or trifocals have.

Propionate- A soft, flexible material that is sometimes used in goggles.

Proximal Vergence- a convergence response attributed to the awareness of, or the impression of nearness of an object of regard. (See "Vergence")

Pseudomyopia- the condition Accommodative Excess/Spasm causes an individual to experience blurry distance vision after prolonged near work such as reading or using a computer. The individual may appear to be nearsighted (myopia). Treatment options may include prescription lenses and/or vision therapy.

Pterygium- Triangular-shaped fold of tissue on white of the eye. May eventually grow over part of the cornea. May be caused by irritation from sun, dust and wind.

Ptosis- droopy upper eyelid, causing the eye to remain partially closed.

Pupil- the opening at the center of the iris of the eye. It contracts (dilates) in the dark and when the eye is focused on a distant object.

Pupillary Distance (PD)- the distances between the pupils of the eyes, in millimeters -- a necessary measurement for proper lens prescription.

Pupillary Reflex- the automatic contraction or enlargement of the pupil when confronted with the presence or absence of light, accommodation, or emotional change.

Pupillometer- a device used to measure the distance between the pupils of the eyes, in millimeters, which is a necessary measurement for proper lens prescription. It also measures the diameter of the pupil.

Pursuit Dysfunction- a condition in which the individual’s ability to follow a moving target is inadequate. Vision therapy is an effective treatment option. To see the American Optometric Association's guidelines for vision therapy, please click here. (See "Ocular Motor Dysfunction")

Pursuit Test- measures the eyes ability to follow a moving target.

Pursuits- the eye’s ability to smoothly follow a moving target.

Radial Keratotomy- (RK) Surgical procedure where cuts are made in the cornea in a radial pattern, to flatten the cornea and correct myopia.

Reading- requires the use of good visual skills, which are distance and near acuity, accommodation skills, binocularity skills (convergence), oculomotor skills (saccadic), peripheral vision, figure-ground, form constancy, spatial relations, visual closure, visual discrimination, visual memory, and visualization.

Refraction Test- determines the eye's refractive error and the best corrective lenses to be prescribed. There are several methods of performing refraction: Retinoscopy, Automated Refractor, and Subjective Refraction.

Refractive Error- defects in vision caused by the eye’s inability to bend, or refract light and focus it clearly on the retina. Astigmatism, hyperopia, and myopia are common conditions of refractive error, also called ametropia.

Refractive Power- a lens' ability to bend parallel light rays into focus, as measured by power diopters. In general, the greater the curvature of a lens and the greater the difference between center thickness and edge thickness, the higher the index of refraction and the greater its refractive power. Refractive power can also refer the strength of a person's contact lenses or glasses.

Refractive Media- the parts of the eye that light travels through before being focused on the retina includes the cornea, crystalline lens, aqueous, and vitreous.

Refractive Surgery- Surgery that corrects visual acuity, with the objective of reducing or eliminating the need for glasses and contacts. Includes radial keratotomy, PRK, LASIK, and corneal implants.

Relative Amblyopia- functional amblyopia can co-exit with a pathology abnormality. Treatment is possible.

Retina- the innermost layer of the eye, a neurological tissue, which receives light rays focused on it by the lens. This tissue contains receptor cells (rods and cones) that send electrical impulses to the brain via the optic nerve when the light rays are present.

Retinal Detachment- Condition where the retina separates from the choroid.

Retinitis Pigmentosa- Condition characterized by the progressive loss of peripheral vision, usually beginning with night blindness.

Retinoscopy- this technique determines the eye's refractive error and the best corrective lenses to be prescribed. An instrument called a retinoscope which consists of a light, lens, mirror, and handle, is used to shine light into a patient’s eye. There are two types of retinoscope: streak and spot retinoscope. When light is shone into patient’s eye, the light is reflected back (“reflex”). If the reflection is in the same direction (“with movement”) of the retinoscope then the refractive error is hyperopia (farsightedness) and a plus lens is prescribed. If the reflection is in the opposite direction (“against movement”) of the retinoscope then the refractive error is myopia (nearsightedness) and a minus lens is prescribed. The strength of the prescription is determined when the pupil is suddenly filled with light (“neutralized”) with the appropriate lens powers (strength). To learn more about the principles of retinoscopy, please click here.  There are other types of retinoscopy such as Static retinoscopy or Dynamic retinoscopy.

Rigid Gas Permeable- (RGP) Type of contact lens made of breathable plastic that is custom-fit to the shape of the cornea. RGPs are the successor to old-fashioned hard lenses, which are now virtually obsolete.

Rod- a receptor cell which is sensitive to light and is located in the retina of the eye. It is responsible for night vision (non-color vision in low level light).

Saccades- the eye's ability to direct and coordinate movement as it quickly and voluntarily shift from one target to another.

Saccades Dysfunction- a condition in which the individual’s ability to scan along a printed page and move his eyes from point to point is inadequate. Symptoms include frequent loss of place while reading, skip or transpose words, and have difficulty comprehending because of an inaccurate eye movement. Vision therapy is an effective treatment option. To see the American Optometric Association's guidelines for vision therapy, please click here. (See "Ocular Motor Dysfunction")   

Saccadic Test- measures the eyes ability to move quickly and precisely from point to point.

Sclera- the white protective covering of the eye.

Slit Lamp(Biomicroscope) – this instrument can examine ocular tissue from the front of the cornea to the back of the lens. A narrow "slit" beam of very bright light produced by a lamp. This beam is focused on to the eye which is then viewed under magnification with a microscope. A joystick control is employed to enable instrument to be moved left-right and up-down. A chin rest, head rest and fixation target is also required. Some slit lamps have a tilting mechanism to enable the lamp to be directed from different angles.

Spatial Relation- the ability to judge the relative position of one object to another and the internal awareness of the two sides of the body. These skills allow the individual to develop the concepts of right, left, front, back, up, and down. This ability is needed in reading and math. (See "Directionality/Laterality")

Sphere- an ophthalmic lens with no cylindrical power or addition. It has the same power in all parts of the lens.

Squint- to be unable to direct both eyes simultaneously toward a point. Also known as strabismus (turned eye). For more information, see "Strabismus".

Stereopsis- the ability to perceive a three dimensional depth which requires adequate fusion (union) of the images from each eye.

Stereopsis Test- measures depth perception that is dependent on the accuracy of eye teaming.

Strabismus- (clinical condition) turned eye (s), the eyes are misaligned. It is caused by a reduction in visual acuity, reduced visual function, high refractive error, traumatic brain injury, oculomotor nerve lesion, or eye muscle injury. In strabismus, the eyes send conflicting images to the brain, and the brain cannot combine these images as it would in normal vision. The brain compensates by ignoring one image in favor of the other, causing a loss of depth perception. Strabismus in more common in children, and affects four percent of all children (although it may also appear later in life).

It is characterized by using the following categories:

• Unilateral strabismus: Strabismus in which only one eye deviates.
• Alternating strabismus: Strabismus in which the deviating eye can change.
• Intermittent strabismus: Strabismus which is not present at all times.
• Constant strabismus: Strabismus which is present at all times.
• Periodic strabismus: Strabismus which occurs at one testing distance but not at another.

Strabismus is also known as squint. It may also be referred to as cross-eyes (convergent- turning inward) or wall eyes (divergent- turning outward). Treatment options may include one or more of the following: optical lenses, bi-focal lenses, prisms, surgery, vision therapy, or Botulinum Toxin Type A (Oculinum, Botox^®) injections.

Stroma- The cornea's middle layer; it consists of lamellae (collagen) and cells, and makes up most of the cornea.

Sty- A blocked gland at the edge of the lid which has become infected by bacteria.

Subconjunctival Hemorrhage– a blood spot on the eye. It occurs when a small blood vessel under the conjunctiva (the transparent coating that covers the inner eyelid and the white of the eye) breaks and bleeds. A common condition caused spontaneously from coughing, heavy lifting, or vomiting. In some cases, it may develop following eye surgery or trauma. It tends to be more common among those with diabetes, hypertension, and taking blood thinners (including aspirin). A subconjunctival hemorrhage is essentially harmless. The blood naturally absorbs within one to three weeks and no treatment is required. If a mild irritation is present, artificial tear drops can be used. You can speed up the healing process by applying cool compresses for the first two days and then warm compresses in the following days.

Subjective Refraction- the procedure in which the patient is asked to report on which lens combination provides the clearest vision. While this is the method of choice for determining prescription in those able to understand the task and respond to the examiner, it is less reliable in children.

Suppression of Binocular Vision- when the brain ignores the image that is seen by one eye. It is the result of weak eye teaming skills (binocularity).

Suppression Test- determines if there is any tendency for the visual processing center of the brain to ignore or suppress visual data from one eye.

Suspensory Ligament- Also called zonule of Zinn. Membrane of fibers (zonules) that holds the eye's lens in place.

Tactile- pertaining to the sense of touch.

Titanium- A type of metal alloy that is very strong. Eyeglasses made of titanium are lightweight, durable and often hypoallergenic.

Tonic- slow, smooth tension. (See "Fusional Vergence")

Tonic Vergence- convergence due to the basic tonicity (tension) of the extraocular muscles, which are responsible, in part, for the distance phoria. Deficient tonic vergence would result in exophoria and excessive tonic vergence results in esophoria. (See "Vergence")

Tonometry- an instrument that measures the pressure within the eye, which is known as intraocular pressure (IOP).

Toric- A lens design with two different optical powers at right angles to each other for the correction of astigmatism.

Trifocal- A lens design that has three focal areas: a lens for close work or reading, a lens for mid-distance viewing or arm's length, and a lens for faraway viewing or driving.

Tunnel Vision- a constriction of the visual field that is commonly caused by chronic glaucoma, retinal degeneration, a tumor, or a brain disorder that interferes with the fibers that connect the optic nerve to the brain. (Please note that a visual stress,  emotional stress, or emotional trauma can also cause a constriction of the visual field.) (See "Streff Syndrome".)

Ultraviolet- (UV) The invisible part of the light spectrum whose rays have wavelengths shorter than the violet end of the visible spectrum and longer than X rays. UVA and UVB light are harmful to your eyes and skin.

Uvea- Middle layer of the eye, below the limbus, and consisting of the iris, ciliary body and choroid.

Uveitis- Inflammation of the uvea.

Vectogram- a three-dimensional picture that is used to strengthen the binocularity system. Available in fixed and variable styles to provide base-in and/or base-out training. 3D glasses are used to view the picture.

Vergence- to turn the eyes horizontally (convergence- inward or divergence- outward). Accommodative vergence, fusional vergence, proximal vergence, and tonic vergence are needed to maintain single vision.

Vergence Facility- a measure of the ease and speed of the eyes to change from a converging to diverging position.

Vertigo- a disordered state in which the individual is dizzy or feels that the surrounding environment is whirling.

Visagraph Eye-Movement Recording System - records and measures eye movements while an individual reads. The system also measures reading efficiency. Specially created goggles and a computer program are used.

Vision-the ability to take in information through our eyes and process the information so that it has meaning.

Vision Therapy (VT) - see "Optometric Vision Therapy"

Vision Therapy Technician- one who works under the supervision of an optometrist in evaluating clients and in planning and implementing vision therapy programs. (See “COVTT”)

Visual Acuity- sharpness or clearness of eyesight. For more information, please click here. (See "Near Acuity" and "Distance Acuity", “20/20”)

Visual Analysis-refers to figure-ground, form constancy, spatial relation, visual closure, visual discrimination, visual memory, and visualization.

Visual Closure- the ability to identify or recognize a symbol or object when the entire object is not visible.

Visual Discrimination- the ability to discriminate between visible likeness and differences in size, shape, pattern, form, position, and color. Such as the ability to distinguish between similar words like "ran" and "run".

Visual Field- the total area that can be seen while looking straight ahead. (See "Tunnel Vision".) (Note: Perimetry is the method of testing an eye's field of vision. For more information, please see "Perimetry".)

Visual Form Dysfunction- difficulty with figure-ground, form constancy, visual closure, and visual discrimination. Symptoms include confusion with similar objects, words, or colors. Vision therapy is a treatment option.

Visual Memory- the ability to recall and use visual information from the past.  (See "Visual Sequential Memory")

Visual Memory Dysfunction- difficulty with retention, recall, or recognition of things seen. Symptoms can include poor spelling and poor recall of visual information. Vision therapy is a treatment option.

Visual-Motor Dysfunction- the inability to process and reproduce visual images by writing or drawing. Symptoms can include poor pencil grip/writing, poor organization on written page, poor copying/spacing, and excessive erasing. Vision therapy is a treatment option.

Visual-Motor Integration (VMI)- after visual data is gathered, it is processed and combined in the brain with information from movement (eye hand coordination).

Visual-Motor Skills-the ability of our eyes to guide our hands (eye hand coordination, visual-motor integration).

Visual Pathway- route of the nerve impulses from the retina along the optic nerve, and optic nerve radiations to the brain's sensory cortex that is located at the base of the skull.

Visual Perceptual Disorders- information processing dysfunctions of the visual system. These dysfunctions can be a directionality/laterality disorder, visual form dysfunction, visual memory dysfunction, and visual-motor dysfunction. Vision therapy is a treatment option. (Also see “Perceptual Skills”)

Visual Skills- are accommodation (eye focusing), binocularity (eye teaming), and oculomotor skills (eye movement), which are neuro-muscular abilities that are controlled by muscles inside and outside of the eye and are networked with the brain.

Visualization-the ability to crate and manipulate mental pictures of an object or concept on the basis of past visual experience and memory. Essential in reading and playing sports.

Vitreous Body- A part of the eye between the lens and the retina, containing a clear jelly called the vitreous humor.

Wandering Eye(s)- see "Exotropia".

Wheatstone Stereoscope- an instrument designed to present separate images to each eye. Each eye can see independently. Two plane mirrors are joined at one edge at a 90-degree angle and two target holders, one opposite one mirror and the other, mounted on a screw base which, when turned, synchronously moves the targets toward or away from each other. The Amblyoscope, Troposcope, Synoptophore, and the Bernell Mirror Stereoscope are examples of this design.

Wraparound- (wrap) Type of eyeglass frame that curves around the head, from the front to the side. Wraparound sunglasses tend to offer extra sun protection because the lenses usually wrap as well.

Yoked Prisms- a wedge-shaped lens which is thicker on one edge than the other. The prism bases (thicker end) are in the same direction for both eyes (up, down, left, or right). Yoked prisms are used to train or compensate for a binocular dysfunction (eye teaming problem) or a visual field loss. Sometimes used in optometric vision therapy programs.