The angle between two radii of a circle such that the length of the arc between them is equal to the radius. The circumference of a circle is equal to 2p times the radius, so one radian contains 360/(2p) = 180/p = approximately 57.2958 degrees. Conversely, multiplying the number of degrees by p/180 gives the number of radians; 360 degrees equals 2p radians.