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 greater noise and impetuosity. The words comes from καταράκτης, "I tumble down with violence;" compounded of κατά, "down," and ράκτης, "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 Piftil Rhaeadr, in North Wales, the fall of water is near 240 feet from the mountain to the lower pool.

Strabo calls that a cataract which we call a cascade; and what we call a cataract, the ancients usually called a catadupa. Herminius has an express dissertation, "De admirandis mundi Cataractis supra et subterraneis;" where he uses the word in a new sense; signifying by cataract, 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, gray, brown, &c. by which vision is variously impeded, or totally destroyed. See Surgery.