The cosine function is a fundamental trigonometric function that relates the angle of a right triangle to the ratio of the length of the adjacent side to the hypotenuse. It is commonly denoted as cos(θ), where θ represents the angle. The cosine function is periodic, meaning it repeats its values in regular intervals, specifically every 360 degrees or 2π radians. In the unit circle, the cosine of an angle corresponds to the x-coordinate of a point on the circle. The values of the cosine function range from -1 to 1, making it useful in various applications, including physics, engineering, and computer graphics.