What is a cosine function?

A cosine function is a mathematical function defined as the ratio of the adjacent side to the hypotenuse in a right triangle, typically denoted as ( \cos(x) ), where ( x ) is the angle in radians. It is a periodic function with a period of ( 2\pi ) that oscillates between -1 and 1. The graph of the cosine function is a wave-like curve that starts at 1 when ( x = 0 ) and decreases to -1, then returns to 1. Cosine functions are widely used in trigonometry, physics, engineering, and signal processing.