/ˈkəʊsaɪn ||; ˈkosaɪn/noun [C] (mathematics数学) (abbr cos) the RATIOof the length of the side next to an ACUTE ANGLE(= one that is less than 90°)to the length of the longest side(the hypotenuse)in a RIGHT-ANGLEDtriangle 余弦（三角形中贴近锐角的一边与斜边之比） ☞Compare sine and tangent. 与sine及tangent比较。