- noun a trigonometric function equal to the length of the side opposite to an angle in a right-angled triangle divided by the hypotenuse
- For a right triangle, the ratio of the length of the side opposite to an acute angle, to the length of the hypotenuse. This contrasts with cosine, which is the ratio of the length of the side adjacent to an acute angle, to the length of the hypotenuse.
Origin & History of “sine”
As in the case of many other mathematical terms, English is indebted to Arabic for sine. But here the debt is only semantic, not formal. The word sine itself was borrowed from Latin sinus ‘curve, fold, hollow’ (source also of English sinuous (16th c.) and indeed of sinus (16th c.), whose anatomical use comes from the notion of a ‘hollow’ place or cavity). In post-classical times it came to denote the ‘fold of a garment’, and so it was mistakenly used to translate Arabic jayb ‘chord of an arc’, a doppelganger of Arabic jayb ‘fold of a garment’.