Opposite-side/hypotenuse ratio (4)

I believe the answer is:

sine

(Other definitions for sine that I've seen before include "Without; maths function" , "Opposite/hypoteneuse" , "Indefinitely, with no appointed date - . . . . die" , "Indispensable condition" , "Mathematical function" .)

I've seen this clue in The Wall Street Journal.

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