A cataract is any opacity which develops in the crystalline lens of the eye or in its envelope. Cataracts form for a wide 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 conditions is relatively simple; the opaque lens is removed, and usually replaced with a synthetic plastic lens. If the lens is not replaced, strong glasses can make up for the loss of focusing ability.
Cataract is also a term used to mean a waterfall or where the flow of a river changes dramatically. It comes from a Greek word meaning "downrush" or "to dash down".
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