Sine
In a right-angled triangle, the sine of an angle is the ratio of the length of the side opposite the angle to the length of the hypotenuse.
Example
For an angle $\theta$, $\sin(\theta) = \frac{\text{Opposite}}{\text{Hypotenuse}}$. If a right-angled triangle has an opposite side of $3$ units and a hypotenuse of $5$ units, then $\sin(\theta) = \frac{3}{5} = 0.6$.