Accommodation: Increase in focusing power of the eye to maintain a clear image as objects are moved closer. Due to the lens becoming rounder. Natural loss of accommodation with increasing age is called presbyopia.

After-cataract or secondary cataract: Remnants of an opaque lens remaining in the eye, or opacities forming, after extracapsular cataract removal.

Age-related macular degeneration or disease (AMD, ARMD) (MAK-yu-lur): Deterioration of the macula affecting people older than 60, resulting in loss of sharp central vision. The leading cause of legal blindness in the United States. View a fact sheet on AMD.

Amblyopia (am-blee-OH-pee-uh) or “lazy eye”: Decreased vision in one or both eyes without detectable anatomic damage to the retina or visual pathways. Usually uncorrectable by eyeglasses. Visit the National Eye Institute Web site for more information. Lazy eye, or amblyopia, is the loss or lack of development of central vision in one eye that is unrelated to any eye health problem and is not correctable with lenses. It can result from a failure to use both eyes together. Lazy eye is often associated with crossed-eyes or a large difference in the degree of nearsightedness or farsightedness between the two eyes. It usually develops before age six and it does not affect side vision.

Amsler grid (AM-zlur): Printed grid (black lines on white background) used as a test for detecting central visual field distortions or defects in macular degenerations. View a fact sheet on age-related macular degeneration.

Angle or anterior chamber angle: Junction of the front surface of the iris and back surface of the cornea, where aqueous fluid filters out of the eye. Important for glaucoma.

Anterior chamber: Fluid-filled space inside the eye between the cornea and the iris. Aqueous (AY-kwee-us) or aqueous humor or aqueous fluid: Clear, watery fluid that fills the anterior and posterior chambers of the eye. Nourishes the cornea, iris, and lens and maintains intraocular pressure.

Astigmatism (uh-STIG-muh-tiz-um): A type of refractive error. Optical defect in which refractive power of an eye is not uniform in all directions (meridians). Large amount may result in headache and significant blurring of images.

Autoimmune diseases (aw-toh-im-YOON) or rheumatoid diseases: Chronic, systemic diseases caused by the immune system’s attack upon some parts of a patient’s body. Several of these diseases -- rheumatoid arthritis, Sjögren's syndrome, systemic lupus erythematosus, and multiple sclerosis -- are associated with serious eye symptoms. For more information, visit the AARDA Web site and also websites of organizations dealing with each disease.

Background retinopathy: See the fact sheet on diabetic retinopathy.

Bifocals: Eyeglasses that incorporate two different refractive powers in each lens, usually for near and distance corrections.

Binocular vision: Blending, by the brain, of the separate images seen by each eye into one composite image.

Blepharitis (blef-uh-RI-tus): Inflammation of the eyelids, usually with redness, swelling, and itching. Blepharitis is a chronic or long-term inflammation of the eyelids and eyelashes. It affects people of all ages. Among the most common causes of blepharitis are poor eyelid hygiene; excessive oil produced by the glands in the eyelid; a bacterial infection (often staphylococcal); or an allergic reaction.

Blind spot: Sightless area within the visual field of a normal eye. Caused by absence of light-sensitive photoreceptors where the optic nerve enters the eye.

Canal of Schlemm: See Schlemm's Canal. Cataracts: Opacity or cloudiness of the crystalline lens, which may prevent a clear image from forming on the retina. Surgical removal of the lens is necessary if visual loss becomes significant, with lost optical power replaced with an intraocular lens.

Cataract extraction: Removal of a cloudy lens from the eye. Extracapsular cataract extraction leaves the rear lens capsule intact; with an intracapsular extraction (usually by cryoextraction) there is complete removal of the lens with its capsule. central vision: An eye's best vision, used for color, reading, and discriminating fine detail. Results from stimulation of the macular area of the retina. A cataract is a clouding of all or part of the normally clear lens within your eye, which results in blurred or distorted vision. Cataracts are most often found in persons over age 55, but they are also occasionally found in younger people

Choroid (KOR-oyd): Major blood vessel (vascular) layer of the eye, lying between the retina and the sclera. Provides nourishment to outer layers of the retina.

Ciliary body (SIHL-ee-uh-ree): A structure near the angle that produces aqueous fluid. See glaucoma.

Color blindness: Reduced ability to discriminate among colors, especially shades of red and green. Usually hereditary; much more common in men.

Cone: Light-sensitive retinal photoreceptor cell that provides sharp visual acuity and color discrimination.

Conjunctiva (kahn-junk-TI-vuh): Transparent mucous membrane covering the outer surface of the eyeball except the cornea, and lining the inner surfaces of the eyelids.

Conjunctivitis (kun-junk-tih-VI-tis) or "pink eye": Inflammation of the conjunctiva. Characterized by discharge, grittiness, redness, and swelling. Usually viral in origin, but may be bacterial or allergic; may be contagious. The three main types of conjunctivitis are infectious, allergic and chemical. The infectious type, commonly called "pink eye" is caused by a contagious virus or bacteria.

Cornea (KOR-nee-uh): Transparent, dome-shaped front part of the eye that covers the iris, pupil, and anterior chamber and provides most of an eye's optical power. Corneal disease: Group of infections, dystrophies, and injuries affecting the cornea. Visit the National Eye Institute Web site for more information.

Cross-eyes: See esotropia.

Diabetic retinopathy (ret-in-AHP-uh-thee): Retinal changes accompanying long-standing diabetes mellitus. Early stage is background retinopathy. May advance to proliferative retinopathy, which includes the growth of abnormal new blood vessels (neovascularization). Leading cause of visual impairment among working-age Americans. Diabetes also can cause cataracts. View fact sheet on diabetic retinopathy. Diabetic Retinopathy Diabetes is a disease that interferes with the body's ability to use and store sugar and can cause many health problems. One, called diabetic retinopathy, can weaken and cause changes in the small blood vessels that nourish your eye's retina, the delicate, light sensitive lining of the back of the eye. These blood vessels may begin to leak, swell or develop brush-like branches.

Dilated pupil: Enlarged pupil, resulting from contraction of the dilator muscle or relaxation of the iris sphincter. Occurs normally in dim illumination, or may be produced by eyedrops containing drugs (mydriatics) during an eye examination.

Diopter (D) (di-AHP-tur): Unit designating the refractive power of a lens. Diplopia, double vision: Perception of two images from one object.

Drusen (DRU-zin): Tiny, white, shiny deposits behind the retina. Common after age 60; sometimes an early sign of macular degeneration.

Dry eye syndrome or dry eye: Corneal and conjunctival dryness due to deficient tear production, predominantly in older women. Can cause foreign body sensation, burning eyes, and erosion of conjunctival and corneal epithelium. View fact sheet on dry eye syndrome..

Emmetropia (em-uh-TROH-pee-uh): Refractive state of having no refractive error. Images of distant objects are focused sharply on the retina without the need for either accommodation or corrective lenses.

Esotropia (ee-soh-TROH-pee-uh), cross-eyes: Eye misalignment in which one eye turns inward (toward nose) while the other fixates normally.

Excimer laser (EKS-ih-mur): Class of ultraviolet lasers that removes tissue accurately without heating it. In refractive corneal surgery, controlled by computer to make precise pre-programmed shavings of eye tissue to produce a given optical correction.

Exotropia (eks-oh-TROH-pee-uh), wall-eyes: Eye misalignment in which one eye turns outward (away from nose) while the other fixates normally.

Extraocular muscles (eks-truh-AHK-yu-lur): Six muscles that move the eyeball. Eyelids: Structures covering the front of the eye, which protect it, limit the amount of light entering the pupil, and distribute tear film over the exposed corneal surface.

Farsightedness: See hyperopia.

Floaters: Particles that float in the vitreous and cast shadows on the retina; seen as spots, cobwebs, spiders, etc. Occur normally in aging and with vitreous detachment, but also pathologically in retinal tears and inflammation. Spots (often called floaters) are small, semi-transparent or cloudy specks or particles within the vitreous, which is the clear, jelly-like fluid that fills the inside of your eyes. They appear as specks of various shapes and sizes, threadlike strands or cobwebs. Because they are within your eyes, they move as your eyes move and seem to dart away when you try to look at them directly

Fluorescein angiography (FLOR-uh-seen an-jee-AH-gruh-fee): Diagnostic technique used for visualizing and recording location and size of eye blood vessels and any problems affecting them. Fluorescein dye is injected into an arm vein; then rapid, sequential photographs are taken of the eye as the dye circulates.

Fovea (FOH-vee-uh): Central pit in the macula that produces sharpest vision. Contains a high concentration of cones and no retinal blood vessels.

Glaucoma (glaw-KOH-muh): Group of diseases, usually characterized by increased intraocular pressure, resulting in damage to the optic nerve. A common cause of preventable vision loss. Leading cause of blindness among black Americans. May be treated by prescription drugs or surgery. Glaucoma is an eye disease in which the internal pressure in your eyes increases enough to damage the nerve fibers in your optic nerve and cause vision loss. The increase in pressure happens when the passages that normally allow fluid in your eyes to drain become clogged or blocked. The reasons that the passages become blocked are not known.

Goblet cells: Goblet-shaped cells in the conjunctiva of the eyelids that produce the mucus of the innermost layer of the tear film.

Hyperopia (hi-pur-OH-pee-uh), farsightedness: Type of refractive error. Focusing defect in which an eye is underpowered; light rays coming from a distant object strike the retina before coming to sharp focus, blurring vision. Corrected with additional optical power, which may be supplied by a plus lens (spectacle or contact).

IOL (intraocular lens): Plastic lens that may be implanted, during cataract surgery, to replace the eye's natural lens.

Intraocular pressure (or tension): Fluid pressure inside the eye. Measured with a tonometer. Usually abnormally high in Glaucoma.

Iris : Pigmented tissue lying behind the cornea that gives color to the eye (e.g., blue eyes) and controls amount of light entering the eye by varying the size of the pupillary opening.

Keratoconus (kehr-uh-toh-KOH-nus): Degenerative, hereditary corneal disease affecting vision. Characterized by generalized thinning and cone-shaped protrusion of the central cornea, usually in both eyes.

Lacrimal gland: Almond-shaped structure that produces the aqueous (watery) middle layer of the tear film. Located at the upper outer region of the eye socket, above the eyeball. Laser: Acronym for Light Amplification by Stimulated Emission of Radiation. High energy light source to cut, burn, or dissolve tissues for various clinical purposes: in the retina, to treat diabetic retinopathy and macular degeneration by destroying leaking and new blood vessels (neovascularization); on the iris or trabecular meshwork, to decrease pressure in glaucoma; after extracapsular cataract extraction, to open the posterior lens capsule.

LASIK (LAY-sik): Acronym for LAser in SItu Keratomileusis. Type of refractive surgery in which the cornea is reshaped to change its optical power. A disc of cornea is raised as a flap, then an excimer laser is used to reshape the middle layer of corneal tissue, producing surgical flattening. Used for correcting myopia, hyperopia, and astigmatism.

"Lazy eye": See amblyopia.

Legal blindness: Best-corrected visual acuity of 20/200 or worse, or reduction in visual field to 20 degrees or less, in the better-seeing eye.

Lens, crystalline lens: The eye's natural lens. Transparent, biconvex-shaped tissue that helps bring rays of light to a focus on the retina.

Low vision: Term usually used to indicate vision of less than 20/200.

Low vision aids: Instruments such as magnifiers, prisms, print and audio materials, and computer programs to allow people with low vision to read and perform other tasks. Help is available even for people who have lost much of their vision. Visit the National Eye Institute Web site for more information.

Macula (MAK-yu-luh): Small central area of the retina surrounding the fovea; area of well-resolved, acute, central vision.

Macular degeneration: Retinal diseases in which central vision is lost. Age-related type (AMD, or age-related macular degeneration) is very common and affects people over 60. Juvenile macular diseases are hereditary; they include Stargardt’s disease and Best's vitelliform macular dystrophy.

Macular hole: A hole in the macula, usually caused by shrinking and pulling of the vitreous. Treated by surgery. Visit the National Eye Institute Web site for more information.

Medical Eye Diseases Anterior Uveitis: Anterior uveitis is an inflammation of the middle layer of the eye, which includes the iris (colored part of the eye) and adjacent tissue, known as the ciliary body. If untreated, it can cause permanent damage and loss of vision from the development of glaucoma, cataract or retinal edema. It usually responds well to treatment; however, there may be a tendency for the condition to recur

Meibomian glands (my-BOW-mee-uhn) or tarsal glands: Glands in the conjunctiva of upper and lower eyelids. Produce the components of the oily, outermost layer of tear film.

Myopia (my-OH-pee-uh), nearsightedness: Focusing defect in which the eye has too much optical power. Light rays coming from a distant object are brought to focus before reaching the retina. Requires a minus lens correction to "weaken" the eye optically and permit clear distance vision. View fact sheet on Myopia. National Eye Institute (NEI): The branch of the U.S. Government's National Institutes of Health devoted to research on the eye and its diseases. Its website, http://www.nei.nih.gov/index.asp, contains useful, unbiased health information.

Nearsightedness: See myopia.

Neovascularization (nee-oh-VAS-kyu-lur-ih-ZAY-shun): Abnormal formation of new blood vessels, usually in or under the retina or on the iris surface. May develop in diabetic retinopathy and age-related macular degeneration.

Nystagmus (ni-STAG-mus): Involuntary, rhythmic, side-to-side or up-and-down (oscillating) eye movements that are faster in one direction than the other.

Ocular (AH-kyu-luhr): Pertaining to the eye and vision.

Ocular Hypertension: Ocular hypertension is an increase in the pressure in your eyes that is above the range considered normal with no detectable changes in vision or damage to the structure of your eyes. The term is used to distinguish people with elevated pressure from those with glaucoma, a serious eye disease that causes damage to the optic nerve and vision loss.

Ophthalmologist (ahf-thal-MAH-loh-jist): Physician (MD) specializing in diagnosis and treatment of refractive, medical and surgical problems related to eye diseases and disorders.

Ophthalmoscope (ahf-THAL-mah-skohp): Diagnostic instrument that allows the doctor to view the retina. Optic disc or optic nerve head: Eyeball end of the optic nerve. Denotes the site of exit of retinal nerve fibers from the eye and entrance of blood vessels into the eye.

Optician (ahp-TISH-un): Professional who makes and adjusts optical aids, e.g., eyeglass lenses, from refraction prescriptions supplied by an opthalmologist or optometrist. Optic nerve: Largest sensory nerve of the eye; carries impulses for sight from the retina to the brain.

Optometrist (ahp-TAHM-uh-trist): Doctor of optometry (OD) specializing in vision problems, treating vision conditions with spectacles, contact lenses, low vision aids and vision therapy, and prescribing medications for many eye diseases. In some states, such as Oklahoma, optometrists may perform minor surgical procedures, as well as laser refractive eye surgery Patching: Covering an amblyopic patient's preferred eye, to improve vision in the other eye.

Perimetry (puh-RIM-ih-tree): A diagnostic test. Method of charting extent of an eye's field of vision with test objects of various sizes and light intensities. Aids in detection of damage to sensory visual pathways. Peripheral vision: Side vision; vision elicited by stimuli falling on retinal areas distant from the macula. Images are not well resolved.

Phacoemulsification (fay-koh-ee-mul-sih-fih-KAY-shun): Surgical procedure. Use of ultrasonic vibration to shatter and break up a cataractous lens, making it easier to remove.

Photophobia (foh-toh-FOH-bee-uh): Abnormal sensitivity to, and discomfort from, light. May be associated with excessive tearing. Often due to inflammation of the iris or cornea.

Photoreceptors: Nerve cells in the retina that are sensitive to light; rods and cones. "pink eye": See conjunctivitis.

Posterior chamber: Fluid-filled space inside the eye between the iris and the lens.

Presbyopia (prez-bee-OH-pee-uh): Refractive condition in which there is a diminished ability to accommodate arising from loss of elasticity of the crystalline lens, as occurs with aging. Usually becomes significant after age 45.

Primary open angle glaucoma (POAG): The most common type of glaucoma. Anterior chamber angle (where the cornea and iris meet) is not plugged; however, aqueous fluid still drains too slowly out of the eye, causing a rise in intraocular pressure.PRK (photorefractive keratectomy) (foh-toh-ruh-FRAK-tihv ke-ruh-TEK-toh-mee). Use of high intensity laser light (e.g., an excimer laser) to reshape the corneal curvature; for correcting refractive errors.

Progressive addition lens (PAL), progressive-power lens: Eyeglass lens that incorporates corrections for distance vision, through midrange, to near vision (usually in lower part of lens), with smooth transitions and no bifocal demarcation line.

Proliterative retinopathy: See diabetic retinopathy.

Ptosis (TOH-sis): Drooping of upper eyelid.

Pupil: Variable-sized, circular opening in the center of the iris that regulates the amount of light entering the eye. Appears black when looking at eye.

Radial keratotomy (RK) (keh-ruh-TAH-tuh-mee): Type of refractive surgical procedure. Series of spoke-like (radial) cuts made in the cornea to flatten it, reducing its refractive power and thereby correcting myopia (nearsightedness). Mostly replaced by LASIK now.

Refraction: A test performed by optometrist or ophthalmologist to determine an eye's refractive error and the best corrective lenses to be prescribed. Series of lenses in graded powers are used to determine which provide sharpest, clearest vision.

Refractive error: Optical defect in an eye. Parallel light rays are not brought to a sharp focus precisely on the retina, producing a blurred retinal image. Can be corrected by eyeglasses (spectacles), contact lenses, or refractive surgery.

Retina (RET-ih-nuh): Light sensitive neural tissue in the back of the eye that converts images from the eye's optical system into electrical impulses. These are sent along the optic nerve to the brain, which interprets them as vision. The retina forms a thin, membranous lining of the rear two-thirds of the eyeball (or globe). Retinal detachment: Separation of the sensory retina from its underlying, supportive, and nourishing retinal pigment epithelium (RPE) cells. Disrupts vision. Usually requires immediate surgical repair. Visit the National Eye Institute Web site for more information.

Retinal pigment epithelium (RPE): Layer of cells behind the sensory retina. Performs many metabolic tasks for the neural retina.

Retinitis pigmentosa (RP): Set of inherited retinal degenerations. Death of photoreceptors gradually leads to night-blindness and to loss of peripheral vision, causing tunnel vision.Retinitis pigmentosa (RP) is a group of inherited diseases that damage the light-sensitive rods and cones located in the retina, the back part of our eyes. Rods, which provide side (peripheral) and night vision are affected more than the cones which provide color and clear central vision

Retinopathy of prematurity (ROP): Retinal disease seen in some low-birthweight, premature babies.

Rod: Light-sensitive, specialized retinal receptor cell that works at low light levels for night vision. A normal retina contains 150 million rods.

Schlemm's Canal (shlemz): Circular channel deep in corneoscleral junction (limbus) that carries aqueous fluid from the anterior chamber of the eye to the bloodstream. Important for glaucoma.

Sclera (SKLEH-ruh): Opaque, fibrous, protective outer layer of the eye ("white of the eye") that is directly continuous with the cornea in front and with the sheath covering optic nerve behind. Sjögren’s syndrome: Autoimmune disease affecting mostly women, in which glands producing tears and saliva are attacked. Causes severe dry eye. For more information, see http://www.sjogrens.org/.

Slit lamp: Microscope used for examining the eye; allows cornea, lens, and otherwise clear fluids and membranes to be seen in layer-by-layer detail.

Snellen chart: Test chart used for assessing visual acuity. Contains rows of letters, numbers, or symbols in standardized graded sizes, with a designated distance at which each row should be legible to a normal eye. Usually tested at 20 ft.

Stargardt’s disease (STAHR-gahrts): Most common type of juvenile macular degeneration.

Strabismus (struh-BIZ-mus): Eye misalignment caused by extraocular muscle imbalance; one fovea is not directed at the same place in a scene as the other.

Sty or stye: Acute, bacterial, pustular infection of oil glands located in an eyelash follicle at the eyelid margin. Tear film: Layered film on the surface of the cornea that keeps the eye lubricated.

Tonometer (tow-NAH-muh-tuhr): Instrument used to measure intraocular pressure.

Trabecular meshwork (truh-BEK-yu-lur): Mesh-like, spongy structure inside the eye at iris-scleral junction of anterior chamber angle. Filters aqueous fluid and controls its flow into Canal of Schlemm's, prior to its leaving the anterior chamber. Often becomes partially blocked in glaucoma.

Trifocal (TRI-foh-kul): Eyeglass lens that incorporates three lenses of different powers. The main portion is usually focused for distance (20 ft.), the center segment for about 2 ft., and the lower segment for near vision (14 in.).

20/20: Normal visual acuity. Upper number is the standard distance (in feet) between an eye being tested and the Snellen eye chart; lower number indicates that a line of letters or symbols of a specific size can be read from 20 feet away. Usher’s syndrome: Inherited condition that causes serious hearing loss plus retinitis pigmentosa (RP). Symptoms usually present at birth or shortly thereafter. SVisit the National Eye Institute Web site for more information.

Uvea or uveal tract (YU-vee-uh): Pigmented layers of the eye (iris, ciliary body, choroid) that contain most of the intraocular blood vessels. Site of uveitis, a serious inflammation

Visual acuity: Assessment of the eye's abi