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. The word comes from καταράσσω, "I tumble down with violence;" compounded of κατά, "down," and ράσσω, ἀβίνιον, "I throw down."—Such are the cataracts of the Nile, the Danube, Rhine, &c. In that of Niagara, the perpendicular fall of the water is 137 feet; and in that of Pistill Rhaeadr, in North Wales, the fall of water is near 240 feet from the mountain to the lower pool.

Strabo calls that a cataraft which we call a cascade; and what we call a cataraft, the ancients usually called a cataufa. Herminius has an express dissertation, "De admirandis mundi Cataractis supra et subterraneis;" where he uses the word in a new sense; signifying, by cataraft, any violent motion of the elements.

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