[sine] n [ML sinus, fr. L, curve] (1593) 1: the trigonometric function that for an acute angle is the ratio between the leg opposite the angle when it is considered part of a right triangle and the hypotenuse
2: a trigonometric function sin A that for all real numbers A is exactly equal to the sine of an angle of measure A in radians and that is given by the sum of a series of which the first term is A and succeeding terms alternate in sign with the kth term being a fraction whose numerator consists of A raised to the exponent 2k - 1 and whose denominator is the factorial of 2k - 1