ra·di·an / ˈrādēən/ • n. Geom. a unit of angle, equal to an angle at the center of a circle whose arc is equal in length to the radius.

radian Angle formed by the intersection of two radii at the centre of a circle, when the length of the arc cut off by the radii is equal to one radius in length. Thus, the radian is a unit of angle equal to c.57.295°, and there are 2π radians in 360°.