From Greek 'katarraktes' (dashing down) — names both waterfalls and the eye condition, linked by 'falling down.'
A large waterfall or a medical condition in which the lens of the eye becomes progressively opaque, resulting in blurred vision.
From Latin cataracta (waterfall; a portcullis), from Greek katarraktēs (a swooping down, a waterfall, a sluice-gate), from katarassein (to dash down, to strike down hard), from kata- (down) + arassein (to strike, to smash). The word entered English in two separate senses: the waterfall sense (14th century) and the medical sense for the eye condition (1540s). The medical metaphor derived from the ancient theory that a membrane or hardened humor falls down within the eye like a portcullis dropping
The same word means both a massive waterfall and a clouded eye lens because medieval doctors believed cataracts were caused by a dark humor 'falling down' inside the eye like a curtain — a medical error that permanently linked thundering waterfalls with failing eyesight in one word.