Source : Webster's Revised Unabridged Dictionary (1913)

Radian \Ra"di*an\, n. [From {Radius}.] (Math.)
   An arc of a circle which is equal to the radius, or the angle
   measured by such an arc.

Source : WordNet®

radian
     n : 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: {rad}]