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]