## Definition of cosine

**:**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

**:**a trigonometric function cos θ that for all real numbers θ is given by the sum of the alternating series {latex}1 - \frac{x^2}{2!} + \frac{x^4}{4!} - \frac{x^6}{6!} + \frac{x^8}{8!} - \dots{/latex} and that is exactly equal to the cosine of an angle of measure θ in radians