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Normal vision. Courtesy NIH National Eye Institute
The same view with a cataract.

A cataract is any opacity which develops in the crystalline lens of the eye or in its envelope. Cataracts form for a variety of reasons, including ultraviolet exposure, secondary effects of diseases such as diabetes, or simply due to advanced age; they are usually a result of denaturation of lens proteins. Genetic factors may also play a role in predisposing someone to cataracts, or conditions of intrauterine development, or injuries to the eye.

Cataracts may be partial or complete, stationary or progressive, hard or soft. Treatment of a cataract which is not complicated by any other eye condition is relatively simple; the opaque lens is removed, and usually replaced with a synthetic lens. If the lens is not replaced, strong glasses or contact lenses can improve the eye's focusing ability.

Human eye cross-sectional view. Courtesy NIH National Eye Institute

Cataract is also used to mean a waterfall or where the flow of a river changes dramatically. It comes from the Greek word meaning "downrush" or "to dash down" or the Latin "cataract" meaning a waterfall, a floodgate or a portcullis.