​CATARACT (from the Lat. form cataracta of the Gr. καταρράκτης, a floodgate, or waterfall, properly something which rushes down), a downpour of water, a waterfall. The earliest use in English is of a floodgate or portcullis, and this survives in the name of a disease of the eye (see Eye: Eye Diseases), in which the crystalline lens becomes opaque, and forms an apparent grating over the eye. The term is also used of a device to regulate the strokes in certain types of steam-engine.