Pronunciation: (kō'sīn),[key] —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 - (x2/2!) + (x4/4!) - + … , where ! denotes factorial. Abbr.: cos Cf. sine (def. 3), factorial (def. 1).