The Collaborative International Dictionary of English v.0.48:

friendly fire \friend"ly fire`\, n. (Mil.)
   Weapons fire[9], such as artillery or aerial bombardment,
   from one's own forces; -- used mostly when troops do damage
   or cause casualties among their own forces; as, the tank was
   hit by friendly fire.
   [PJC]

WordNet (r) 3.0 (2006):

friendly fire
    n 1: fire that injures or kills an ally [syn: friendly fire,
         fratricide]