ˈrādēən noun
( -s )
Etymology: radi us + -an
: a unit of plane angular measurement equal to the angle at the center of a circle subtended by an arc equal in length to the radius
since the circumference of a circle is equal to 2π times its radius, the number of radians in an angle of 360 degrees or in a complete turn is 2π, so that 1 radian equals about 57.29 degrees