A cosine function is a mathematical function that describes the relationship between the angle of a right triangle and the lengths of its sides. It is defined as the ratio of the length of the adjacent side to the hypotenuse. The cosine function is commonly denoted as cos(x), where x is the angle measured in radians or degrees. Cosine functions are periodic, meaning they repeat their values in regular intervals. The graph of a cosine function produces a wave-like pattern, oscillating between -1 and 1. This behavior makes cosine functions useful in various fields, including physics, engineering, and signal processing.