[co.sine] n [NL cosinus, fr. co- + ML sinus sine] (1635) 1: a trigonometric function that for an acute angle is the ratio between the leg adjacent to the angle when it is considered part of a right triangle and the hypotenuse
2: a trigonometric function cos A that for all real numbers A is given by the sum of a series of which the first term is 1 and succeeding terms alternate in sign with the kth term consisting of a fraction whose numerator is A raised to the exponent 2(k - 1) and whose denominator is the factorial of 2(k - 1) and that is exactly equal to the cosine of an angle of measure A in radians