cataract (n.)

early 15c., "a waterfall, floodgate, furious rush of water," from Latin cataracta "waterfall," from Greek katarhaktes "waterfall, broken water; a kind of portcullis," noun use of an adjective compound meaning "swooping, down-rushing," from kata "down" (see cata-). The second element is traced either to arhattein "to strike hard" (in which case the compound is kat-arrhattein), or to rhattein "to dash, break."

Its alternative sense in Latin of "portcullis" probably passed through French and gave English the meaning "eye disease characterized by opacity of the lens" (early 15c.), on the notion of "obstruction" (to eyesight). Related: Cataractous.