A cataract is a clouding of the normally clear and transparent lens of the eye. It is not a tumor, a new growth of skin or tissue over the eye, but a fogging of the lens itself. Metabolic changes of the crystalline lens fibers over the time lead to the development of the cataract and loss of transparency.

The normal process of aging may cause the lens to harden and turn cloudy. These are called senile cataracts and are the most common type. Eye injuries can cause cataracts at any age. Certain infections or diseases of the eye can also cause the lens to cloud and form a secondary cataract.

If a large portion of the lens becomes cloudy, sight can be partially or completely lost until the cataract is removed. 

During cataract surgery the cataract lens is removed and an artificial intraocular lens implant is inserted (eye surgeons say that the lens is "implanted").

Cataract surgery is generally performed by an ophthalmologist (eye surgeon) at an ambulatory (rather than inpatient) setting, in a surgical center or hospital, using local anesthesia. Well over 90% of operations are successful in restoring useful vision, with a low complication rate.

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