Radian

A unit of angular measure in which the Angle of an entire Circle is $2\pi$ radians. There are therefore 360° per $2\pi$ radians, equal to $180^\circ/\pi$ or 57.29577951°/radian. A Right Angle is $\pi/2$ radians.

See also Angle, Arc Minute, Arc Second, Degree, Gradian, Steradian