The cosine function, often abbreviated as cos, is a fundamental concept in trigonometry. It relates the angle of a right triangle to the ratio of the length of the adjacent side to the hypotenuse. For any angle θ, the cosine value can be found using the formula: cos(θ) = adjacent/hypotenuse. This function is periodic, meaning it repeats its values in regular intervals, specifically every 360 degrees or 2π radians. In addition to its geometric interpretation, cosine has important applications in various fields, including physics, engineering, and computer graphics. The unit circle is a key tool for understanding cosine, as it allows for the visualization of cosine values for all angles. The function is also used in Fourier analysis to represent periodic functions as sums of sine and cosine waves.