/koh"suyn/ , n.
1. Trigonom.
a. (in a right triangle) the ratio of the side adjacent to a given angle to the hypotenuse.
b. the sine of the complement of a given angle or arc. Abbr.: cos
2. Math. (of a real or complex number x ) the function cos x defined by the infinite series 1 - ( x 2 /2!) + ( x 4 /4!) - + ... , where ! denotes factorial. Abbr.: cos Cf. sine (def. 3), factorial (def. 1).
[ 1625-35; cosinus. See CO-, SINE 1 ]