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Accommodation
The ability of the eye to change its focus from distant to near objects. This process is achieved by the lens changing its shape.

Age-Related Macular Degeneration
Loss of central vision as a result of pathological changes, often related to aging, in the macula (the back portion of the retina responsible for clear, sharp vision).

Amblyopia (Lazy Eye)
The loss or lack of central vision in one eye, unrelated to eye disease. Usually occurs by age four and involves a lack of vision development generally related to a failure to use both eyes together.

Ametropia
Any optical error that can be corrected by eyeglasses or contact lenses.

Anterior Chamber
The space in front of the iris and behind the cornea.

Anti-Reflective Coatings
Lens coatings that reduce reflection off lenses for cosmetic improvement; also reduce glare and enhance vision.

Aqueous Fluid
Clear, watery fluid that flows between and nourishes the lens and the cornea. Also known as the aqueous humor.

Aspheric Lenses
Flatter and thinner than conventional lens design, aspheric lenses minimize lens magnification and improve the appearance of the eyes behind glasses, especially for people with strong prescriptions.

Astigmatism
A condition that occurs when the front surface of the eye, the cornea, is slightly irregular in shape. This irregular shape prevents light from focusing properly on the back of the eye (the retina). As a result, vision may be blurred at all distances.

Bifocal Lenses
Eyeglasses with two viewing zones, one for seeing near objects and one for distance. Bifocal lenses have a well-defined single visible line separating the two viewing zones.

Blind Spot
(1) A small area of the retina where the optic nerve enters the eye; occurs normally in all eyes. (2) Any gap in the visual field corresponding to an area of the retina where no visual cells are present.

Cataract
A clouding of part or all of the lens inside the eye, resulting in blurred or distorted vision.

Choroid
The layer filled with blood vessels that nourishes the retina; part of the uvea.

Computer Vision Syndrome (CVS)
The American Academy of Optometry defines CVS as "the complex of eye and vision problems related to near work which are experienced during or related to computer use."

Conjunctiva
The thin, moist tissue (membrane) that lines the inner surfaces of the eyelids and the outer surface of the sclera (the outer layer of the eyeball).

Conjunctivitis (Pink Eye)
An irritation/inflammation of the lining of the eyelid and the transparent tissue covering the white part of the eye (conjunctiva). There are two types: bacterial and viral

Contrast Sensitivity
The ability to perceive differences in brightness between an object and its background.

Cornea
The front transparent part of the outer protective layer of the eye. Its bulging curvature provides the major refraction of the eye.

Crossed-Eyes: See Strabismus

Diabetic Retinopathy
The weakening of the small blood vessels in the retina, resulting in blood leakage, growth of new blood vessels and other changes; caused by diabetes. Vision may be blurred, cloudy or otherwise distorted and may be lost if left untreated.

Dilation
A process by which the pupil is temporarily enlarged with special eye drops. This allows an eye doctor to see more of the retina and look for signs of diabetic retinopathy and other eye diseases or health concerns. After the examination, close-up vision may remain blurred for several hours.

Emmetropia
The state of having no refractive error.

Farsightedness: See Hyperopia

Floaters (Spots)
Small semi-transparent or cloudy particles floating in the vitreous, the transparent, colorless mass of gel that lies behind the lens and in front of the retina.

Fundus
The interior lining of the eyeball, including the retina, optic disc, and macula. This portion of the inner eye can be seen during an eye examination by looking through the pupil.

Glass Lenses
Glass lenses, made primarily from silica, are the oldest type of prescription lens. They are heavier than plastic lenses, and must be hardened by heat or chemicals to meet U.S. Food and Drug Administration standards for impact resistance.

Glaucoma
An eye disease in which the passages that allow fluid in the eye to drain become clogged or blocked, or there is too much fluid produced inside of the eye. Increased pressure inside the eye then damages the optic nerve and causes vision loss.

Hard Resin Lenses (Plastic Lenses)
Plastic (hard resin) lenses are lighter and more impact resistant than glass lenses, but are more easily scratched. Wearers usually prefer the advantages of a high-index lens.

High-Index Lenses
High-index lenses are the newest type of prescription eyeglass lens. For high prescriptions, they are lighter in weight and thinner at the edge than regular glass and plastic lenses. Available in both glass and plastic, the latter being more popular. Scratch-resistant and anti-reflection coatings are often recommended.

Hyperopia (Farsightedness)
The inability to see objects up close. It is the result of an eyeball that is too short or whose outside surface (the cornea) is too flat. The exact cause is not known, although farsightedness may be inherited, occur naturally, or be associated with eyestrain. It is correctable with single-vision lenses.

Intraocular Pressure (IOP)
Pressure of the fluid inside the eye.

Iris
The colored ring of tissue suspended behind the cornea and immediately in front of the lens. It regulates the amount of light entering the eye by adjusting the size of the pupil.

Lacrimal Gland
The small almond-shaped structure that produces tears. It is located just above the inner corner of the eye.

Lazy Eye: SeeAmblyopia

Lens
The transparent, crystalline, flexible structure in the front of the eye directly behind the pupil. Tiny muscles change the shape of the lens to bend the entering light, helping the eye focus for seeing up close and far away.

Legal Blindness
In the United States, (1) visual acuity of 20/200 or worse in the better eye with corrective lenses (20/200 means that a person must be at 20 feet from an eye chart to see what a person with normal vision can see at 200 feet), or (2) visual field restricted to 20 degrees diameter or less (tunnel vision) in the better eye.

Low Vision
The partial loss of vision due to an eye injury or an acquired, congenital, or inherited eye disorder. It may result in a reduced ability to see clearly at far or near distances, or a reduced or partial field of vision. In older adults, it is often the result of macular degeneration, glaucoma, cataracts, or diabetes. About one in every 20 Americans has low vision; a variety of specialized optical aids and services helps them continue to work, read, drive, use computers, and enjoy hobbies.

Macula
The small, sensitive area of the central retina that contains light sensitive nerve cells called "cones." It provides vision for fine work and reading.

Macular Degeneration
Loss of central vision as a result of changes, often related to aging, in the macula (the back portion of the retina responsible for clear, sharp vision).

Myopia (Nearsightedness)
The inability to see clearly at a distance. It is caused by an eyeball that is too long or whose outside surface (the cornea) is too curved. Nearsightedness can be inherited, occur naturally, or be caused by the stress of concentrating for long periods on work at a close distance. The condition is correctable with single-vision lenses.

Nearsightedness: See Myopia

Ophthalmologist
Medical doctor specializing in the eye; has completed medical school. Licensed to examine eyes, treat eye diseases and perform eye surgery.

Ophthalmoscopy
An examination of the retina in which the eye doctor, (1) looks through a device with a special magnifying lens that provides a narrow view of the retina, or (2) looks through a special magnifying glass and gains a wide view of the retina.

Optic Nerve
The bundle of more than a million nerve fibers that carry visual messages from the retina to the brain.

Opticianry
Opticianry is the health care profession which analyzes and interprets optical prescriptions in order to design, modify, prepare, fit, verify and deliver ophthalmic devices and assists in other ancillary eye care procedures.

Optometrist
Doctors of optometry are independent primary health care providers who examine, diagnose, treat and manage diseases and disorders of the visual system, the eye and associated structures as well as diagnose related systemic conditions.

Peripheral Vision
Side vision. The ability to see objects and movement outside of the direct line of vision.

Photochromic Lenses
Lenses that darken in bright sunlight and clear in dim light.

Pink Eye: SeeConjunctivitis

Polycarbonate
Lenses designed for impact resistance. Polycarbonate is anywhere from four to ten times as impact resistant as any other lens; in fact, it is used for bulletproof windows. It is light and thin like other high-index lenses, but scratches easily. As a result, a scratch-resistant coating is required. Recommended for children's eyewear and for sports.

Posterior Chamber
The space between the back of the iris and the front face of the vitreous. It is filled with aqueous fluid.

Presbyopia
The gradual loss of the eye's ability to change focus for seeing near objects. It happens because, with age, the lens inside the eye gradually loses its flexibility and focusing ability. It occurs in almost all people over age 45. The condition is correctable with bifocals or progressive addition lenses.

Progressive Addition Lenses
Lenses that have progressively more reading power from top to bottom; for correcting presbyopia.

PRK
Surgical procedure to reshape the cornea with excimer laser; approved for use in correcting myopia; also known as laser vision correction.

Pupil
The adjustable opening at the center of the iris that allows varying amounts of light to enter the eye.

Refractive Surgery
Any one of various surgical procedures to reshape the cornea and correct vision, including PRK, LASIK, and RK.

Retina
The light-sensitive layer of nerve cells (rods and cones) that lines the back of the eyeball. It sends visual impulses through the optic nerve to the brain. The macula is in the center of the retina and is made up of cones.

Retinal Pigment Epithelium
The pigment cell layer that nourishes the retinal cells. It is located just outside the retina and attached to the choroid.

Sclera
The tough, white, outer layer (coat) of the eyeball. Along with the cornea, it protects the entire eyeball.

Scratch-Resistant Coatings
Hardened coatings to protect plastic lenses from scratching.

Single Vision Lenses
Eyeglass lenses that correct a single refractive need (e.g. for myopia or hyperopia).

Spots (Floaters)
Small semi-transparent or cloudy particles floating in the vitreous, the transparent, colorless mass of gel that lies behind the lens and in front of the retina.

Strabismus (Crossed-Eyes)
A misalignment of the eyes in which one or both may turn in, out, up, or down.

Tetrahydrozoline
Ingredient in some eye drops that can irritate an eye with conjunctivitis ("pink eye").

Tonometry
A standard test that determines the fluid pressure inside the eye. Elevated pressure is a possible sign of glaucoma.

Trifocals
Eyeglasses with three different viewing areas to provide clear vision at near, distance and mid-range; trifocals have two well-defined visible lines separating the viewing zones.

Ultraviolet Rays
Invisible rays from sunlight and lamps; potentially harmful to eyes, especially related to the development of cataracts.

Visual Acuity
The ability to distinguish details and shapes of objects.

Visual Acuity Test
A test that assesses visual acuity, often using a test chart of known size at 20 feet.

Visual Field
The entire area that can be seen when the eye is looking straight ahead, including peripheral side vision.

Vitreous
The transparent, colorless mass of gel that lies behind the lens and in front of the retina. Small semi-transparent or cloudy particles floating in the vitreous are called spots or floaters.