Cosine
In a right-angled triangle, the cosine of an angle is the ratio of the length of the adjacent side to the length of the hypotenuse.
Example
For an angle $\theta$, $\cos(\theta) = \frac{\text{Adjacent}}{\text{Hypotenuse}}$. If the adjacent side is $4$ units and the hypotenuse is $5$ units, then $\cos(\theta) = \frac{4}{5} = 0.8$.