Cosine

Cosine is one of the two basic trigonometric functions, alongside sine, that describe the relationship between the angles and side lengths of a right triangle. For an acute angle in a right triangle, the cosine is defined as the ratio of the length of the side adjacent to the angle to the length of the hypotenuse. Its definition has since been extended beyond triangles through the unit circle, letting it take any real number as an angle, and further through infinite series and differential equations, letting it take complex numbers as well. Because it rises and falls smoothly and repeats forever, cosine is used to model periodic phenomena such as sound waves, light waves, and the swing of an oscillator, and its roots trace back to the jya and koti-jya functions of Gupta-era Indian astronomy. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/