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Glossary of Terms

Common terms to help you learn more about your eyes and vision

20/20 Vision

The ability to correctly perceive an object or letter of a designated size from a distance of 20 feet; normal visual acuity.


Ability of the lens to adjust its shape for vision at various distances in order to produce a clear image on the retina.

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 serious and may include blurred vision, severe headaches, eye pain, nausea, vomiting or seeing rainbow-like halos around lights. Occasionally, the condition may be without symptoms;

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.


Irregular curvature of the cornea or lens resulting in a distorted image because light rays are not focused on a single point on the retina.


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.

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).