- A function of an angle expressed as the ratio of two of the sides of a right triangle that contains that angle; the sine, cosine, tangent, cotangent, secant, and cosecant. Also called circular function 三角函数：角的函数，用直角三角形夹该角的两条边的比表示；正弦、余弦、正切、余切、正割、余割 也作 circular function