From WordNet (r) 3.0 (2006) :
rad n 1: a unit of absorbed ionizing radiation equal to 100 ergs per gram of irradiated material 2: 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]