noun. the unit of plane angle adopted under the Systeme International d'Unites; equal to the angle at the center of a circle subtended by an arc equal in length to the radius (approximately 57.295 degrees).
1. Adjective The radian measure of an angle is the ratio between the length of the arc opposite the angle and the radius of the circle. 2. Noun, singular or mass A radian is defined as the angle formed by a line that scribes an arc length on the circumference of a circle equal to the radius of the circle.