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Found one definition

1.                 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]