n.
Pronunciation: ' k ō - ˌ s ī n
Function: noun
Etymology: New Latin cosinus, from co- + Medieval Latin sinus sine
Date: 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 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 cosine of an angle of measure θ in radians