Radian

A radian is a unit of measurement for angles. In a circle, the measure of a central angle in radians is the ratio of the length of the intercepted arc to the length of the circle's radius.

In a unitcircle, a complete angle has measure $2\pi$ , since a complete angle "intercepts" the whole circumference of the circle. Thus, radians can be converted to degrees: $2\pi\; rad=360^\circ\implies\pi \;rad=180^\circ$ .