I. ˈkō+ˌ- noun
( -s )
Etymology: New Latin cosinus, from Medieval Latin sinus sine
: the abscissa of any point, except the vertex, on the terminal side of an angle, divided by the distance between the vertex and the point, the vertex coinciding with the origin of a plane rectangular coordinate system and the initial side of the angle coinciding with the positive x-axis — abbr. cos
II. noun
: 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