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) [syn: radian, rad]
Radian \Ra"di*an\ (r[=a]"d[i^]*an), n. [From Radius.] (Math.)
An arc of a circle which is equal to the radius, or the angle
measured by such an arc.
[1913 Webster] Radiance
