cataract: [15] Greek kataráktēs meant literally ‘swooping down, rushing down’; it was a derivative of the verb katarássein, a compound formed from the prefix katá- ‘down’ (which appears in a wide range of English words, including cataclysm, catalepsy, catalogue, catapult – literally ‘hurl down’ – and catastrophe) and the verb rássein ‘strike’.
Hence it was applied metaphorically to various things that ‘rush down’, including waterfalls and portcullises. The word passed into English via Latin cataracta, and the sense ‘opacity of the eye’s lens’ developed in the 16th century, probably as a metaphorical extension of the now obsolete ‘portcullis’.
cataract (n.)
early 15c., "a waterfall, floodgate," 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 was passed through French to form the English meaning "eye disease" (early 15c.), on the notion of "obstruction" (to eyesight).