COSINE

noun Etymology: New Latin cosinus, from co- + Medieval Latin sinus sine Date: 1635 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, a trigonometric function cos θ that for all real numbers θ is given by the sum of the alternating series cos θ = 1 - θ 2 /2! + θ 4 /4! - θ 6 /6! + θ 8 /8! - . . . and that is exactly equal to the ~ of an angle of measure θ in radians