Tangent
In a right-angled triangle, the tangent of an angle is the ratio of the length of the opposite side to the length of the adjacent side.
Example
For an angle $\theta$, $\tan(\theta) = \frac{\text{Opposite}}{\text{Adjacent}}$. If the opposite side is $3$ units and the adjacent side is $4$ units, then $\tan(\theta) = \frac{3}{4} = 0.75$.