The ability to correctly perceive an object or letter of a designated size from a distance of 20 feet; normal visual acuity.
Glossary of Eye and Vision Terms
Common terms to help you learn more about your eyes and vision
Accommodation
Ability of the lens to adjust its shape for vision at various distances in order to produce a clear image on the retina.
Acute Angle Closure Glaucoma
Those of Asian and Native American descent are at higher risk for acute (angle closure) glaucoma. It occurs when the drainage system of the eye becomes blocked. It causes a
Angle Closure Glaucoma
Acute (Angle Closure) Glaucoma occurs when the drainage system of the eye becomes blocked. It causes a sudden rise in pressure, requiring immediate, emergency medical care. The signs are usually
Annular Solar Eclipses
Annular solar eclipses are often described as a “ring of fire” because a thin outer layer of the sun’s disk is not completely covered by the moon when the moon
Anterior Chamber
Space in front portion of the eye between the cornea and the iris and lens, which is filled with aqueous humor.
Aqueous Humor
Clear, watery fluid that fills the anterior chamber and the posterior chamber in the front part of the eye and provides nutrients to structures in the anterior chamber.
Aqueous Outflow Pathway
The main exit route of aqueous humor from the eye, including the trabecular meshwork and Schlemm’s canal.
Astigmatism Definition
Astigmatism (uh-STIG-muh-tiz-um) is a common and generally treatable vision issue when the cornea (clear part in the front of the eye) is not completely round. It is curved more like
Binocular Vision
Coordinated use of the two eyes to see a single fused three-dimensional image.
Blindspot
In testing the visual field, this is the blind area corresponding to the optic disk where the optic nerve fibers exit the eye and where there are no light-sensitive cells.
Bulging Eye Disease
Bulging eyes are a symptom of thyroid Eye Disease (TED), sometimes called Graves’ ophthalmopathy or Graves’ Eye Disease, is an autoimmune disease in which the immune system causes inflammation and
Central Visual Field
The area of objects in space seen without moving the head or eyes; corresponds to an area within 30º of the fixation point (fovea).
Choroid
The middle layer of the eye containing blood vessels that furnish nourishment to the other parts of the eye, especially the retina.
Ciliary Body
A ring of tissue between the iris and the choroid consisting of muscles and blood vessels that changes the shape of the lens and manufactures aqueous humor.
Coats’ Disease
Coats’ disease is a rare eye problem usually affecting only one eye. The disease is not hereditary, mostly occurring in young males, usually before age 15, but can occur later
Color Vision Deficiency (Color Blindness)
Inability to recognize certain colors, primarily red or green, but rarely blue.
Cones
Cone-shaped light-sensitive cells in the retina particularly in the macula area; cone function predominates in daylight with a small pupil allowing one to make out details and shapes, especially colors.
Congenital
Present at birth.
congenital glaucoma
Congenital/childhood/pediatric glaucoma is glaucoma diagnosed early in a child’s life, as infants or young children. It causes increased intraocular pressure that can damage the optic nerve in the back of
Conjunctiva
Delicate tissue that lines the inside of the eyelids and covers the front part of the eye except the cornea.
Cornea
The clear curved structure that comprises the front of the eye, a refractive surface through which light enters.
Cross eyed
“Cross eyed” is sometimes used to describe a misalignment of the eye. The misalignment results from the failure of the eye muscles to work together. One eye, or sometimes both,
Degeneration
Tissue changes that make it less able to carry out its function.
Depth Perception
The blending of slightly dissimilar images from the two eyes for the perception of three–dimensional depth.
Detached Retina
Vitreous fluid, the gel-like material that fills the eyeball, is attached to the retina around the back of the eye. If the vitreous changes shape, it may pull away a
Diabetic Retinopathy
Diabetic Retinopathy or Diabetes-related retinopathy occurs when small blood vessels leak and bleed in the retina. The retina is layer of the eye that acts like the film in the
Diopter
Metric unit used to denote the refractive error of the eye or lens.
Distance Vision
Ability to distinctly perceive objects at a distance, usually tested at 20 feet.
Eclipse Glasses
To view a solar eclipse safely and without damaging your eyes, solar eclipse glasses can help. Make sure they meet safety requirements and are manufactured with the ISO 12312-2 standard.
Eye Pressure
Eye pressure is known as intraocular pressure (IOP). That is a measure of the pressure inside your eyeball. Find out more about high intraocular pressure
Farsightedness
Also called hyperopia; a refractive error in which the focal point for light rays is behind the retina; distant objects are seen more clearly than near objects. Find out more
Fovea
A tiny spot located in the macula that is the area of clearest vision on the retina.
Hereditary
Appearing in, or characteristic of, successive generations; individual differences in human beings passed from parent to offspring.
Incidence
Number of new cases of a particular problem or disease that occurs within a period of time.
Iris
Colored circular membrane that is in front of the lens and controls the size of the opening at its center (pupil), thereby regulating the amount of light entering the eye.
keratitis acanthamoeba
Acanthamoeba keratitis is sometimes referred to incorrectly as “keratitis acanthamoeba.” Find out more about acanthamoeba keratitis
Laser
Surgical tool using an intense beam of light energy to close rips, make holes, destroy new vessels (photocoagulation), or to open channels as in the treatment of glaucoma.
Legal Blindness
Visual acuity that does not exceed 20/200 in the better eye with correcting lens; field of vision no greater than 20 degrees in its widest angle (Visual acuity of 20/200
Lens
The transparent disc in the middle of the eye behind the pupil that brings rays of light into focus on the retina.
Low-Vision Aids
Powerful optical devices useful to persons with vision impairments that are not successfully corrected by the usual prescription lenses.
Macula
The region of the retina that helps provide best central vision. The fovea is at the center of the macula.
Matricular degeneration
This is a common misspelling of macular degeneration. Find out more about age-related macular degeneration
Myopia
Myopia (my-OH-pea-uh), or nearsightedness, is a vision condition in which distant objects appear blurred – such as roadway signs, the board at school, faces, or the television set across the
Near Vision
The ability to perceive objects distinctly at normal reading distance (usually about 14 inches from the eye).
Nearsightedness
Myopia (my-OH-pea-uh), or nearsightedness, is a vision condition in which distant objects appear blurred – such as roadway signs, the board at school, faces, or the television set across the
Night Blindness
Condition in which sight is good by day but deficient at night and in any faint light.
Optic Disk
Head of optic nerve; formed by the meeting of all retinal nerve fibers in the retina.
Optic Nerve
Special nerve of sight beginning in the retina as the optic disk, which carries messages from the retina to the brain, resulting in visual images.
Partial Solar Eclipses
Partial solar eclipses occur when the moon passes in front of the sun, but only blocks part of the sun. Viewing a partial solar eclipse can expose your eye to