__ qua non (4)
I believe the answer is:
sine
(Other definitions for sine that I've seen before include "Trigonometrical function" , "Trig function" , "Without; maths function" , "Trigonometric function" , "Opposite/hypoteneuse" .)
I believe the answer is:
sine
(Other definitions for sine that I've seen before include "Trigonometrical function" , "Trig function" , "Without; maths function" , "Trigonometric function" , "Opposite/hypoteneuse" .)