Origins
The word 'cataract,' used for both a large waterfall and an eye condition causing progressive opacitβββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββββy of the lens, entered English in the fifteenth century from Latin 'cataracta' (a waterfall; a portcullis, or gate that rushes down), from Greek 'katarraktΔs' (a swooping down, a rushing down, a waterfall, a portcullis). The Greek word derives from 'katarassein' (to dash down), composed of 'kata-' (down) and 'arassein' (to strike, to smash, to dash). At its etymological core, a cataract is something that strikes or dashes downward with force.
The dual meaning of 'cataract' β waterfall and eye disease β has puzzled English speakers for centuries, but the connection lies in a medieval medical metaphor. The eye condition was named in the sixteenth century (first attested around 1540) based on the then-prevailing theory that cataracts were caused by a morbid humor 'falling down' within the eye, creating an opaque curtain or barrier across the field of vision. The image was of a portcullis β a gate that drops down to block a passage β or of a waterfall cascading over the visual field. Medieval Arabic physicians used the term 'nuzΕ«l al-mΔΚΎ' (descent of water) for the condition, and the Latin/Greek 'cataracta' may have been chosen as a translation of this Arabic medical term.
Modern ophthalmology has shown that cataracts actually result from the gradual accumulation of damaged proteins in the eye's lens, causing it to become cloudy β a process of slow aggregation, nothing like the dramatic 'falling down' that the medieval etymology implies. Yet the word persists, a monument to an obsolete theory of disease, because language does not update its vocabulary when science revises its models. English is full of such fossils: 'lunatic' (from the belief that madness was caused by the moon), 'disaster' (from the belief that catastrophes were caused by unfavorable stars), 'influenza' (from the belief that epidemics were caused by celestial 'influence').
Development
The 'waterfall' sense of 'cataract' tends to denote particularly large, powerful falls β the word implies overwhelming volume and force rather than delicate cascading. The Cataracts of the Nile, the series of shallow rapids and waterfalls along the Nile River in Sudan and Egypt, have been known by this name since antiquity. The first Western explorers to encounter Niagara Falls in the seventeenth century immediately applied 'cataract' to describe the overwhelming spectacle of water plunging over the escarpment.
The Greek prefix 'kata-' (down, against, completely) is one of the most productive in English. It appears in 'catastrophe' (a down-turning, an overturning), 'catalogue' (a counting down), 'category' (an accusation, literally 'speaking against' in the assembly), 'cathedral' (from 'kathedra,' a sitting down, a chair β the bishop's seat), 'catapult' (a hurling down), and 'cataclysm' (a washing down, a deluge). Each word preserves the downward or destructive force that 'kata-' imparts, the sense of something moving powerfully and irresistibly from high to low.
The word 'cataract' thus stands at the intersection of natural observation and medical metaphor, preserving both the ancient awe at the spectacle of falling water and the medieval physician's attempt to explain blindness through the image of an impenetrable curtain dropping across the window of the eye.