A cataract is a clouding of the eye's lens, which causes interference with sight. The majority of catacacts are caused by aging, which triggers chemical changes in the eye's natural lens. The age at which this occurs varies greatly, with some people being affected in their fifties, some in their late eighties. Cataracts can also be caused by metabolic changes, injury, radiation or toxic chemicals.

The word "Cataract" comes for the Greek for waterfall. Until the mid 1700s cataracts were believed to be caused by the flow of opaque materials into the eye.