<p><strong>Noun</strong></p><ul><li>In a right triangle, the ratio of the side adjacent to a given angle to the hypotenuse.</li><li>The sine of the complement of a given angle or arc.</li><li>Mathematics: (of a real or complex number x) the function cos x defined by the infinite series 1 − (x²/2!) + (x⁴/4!) − …, where ! denotes factorial.</li></ul>