/saɪn ||; saɪn/noun [C] (abbr sin) (mathematics数学) the RATIO of the length of the side opposite one of the angles in a RIGHT-ANGLED triangle (= a triangle with one angle of 90°) to the length of the longest side 正弦（三角函数之一，指直角三角形某锐角的对边与斜边之比） ☞Compare cosine and tangent. 与cosine及tangent比较。