From The Collaborative International Dictionary of English v.0.48 :
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
From WordNet (r) 3.0 (2006) :
radian
n 1: 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]