The ratio between the lengths of the adjacent side and the hypotenuse in a right triangle for a specific angle, $\theta ,$ is called the cosine of $\theta$ and is written as $\cos (\theta ).$

The ratio is always the same for any given angle. If the hypotenuse is, for example, twice as long as the adjacent side, the ratio is $\cos (\theta )=\frac{1}{2}.$ This quotient holds true because the triangles are bound by similarity. Cosine only states the ratio between the adjacent side and the hypotenuse, it gives no indication about the lengths of the individual sides.