in hydrography, a precipice in the channel of a river, caused by rocks, or other obstacles, stopping the course of the stream, from whence the water falls with a greater noise and impetuosity: Such are the cataracts of the Nile, the Danube, Rhine, and the famous one of Niagara in America.

in medicine and surgery, a disorder of the humours in the eye, by which the pupilla, that ought to appear transparent and black, looks opaque, grey, blue, brown, &c., by which vision is variously impeded, or totally destroyed. See Medicine, and Surgery.