The Collaborative International Dictionary of English v.0.48:

infighting \infighting\ n. [in- + fighting.]
   1. contention among members of the same organization (usually
      concealed from outsiders), sometimes of a bitter, intense,
      or prolonged character.

   Note: [Such infighting may lead to factionalism.] PJC]

   2. fighting, such as boxing, at close quarters.
      [PJC]

   3. rough-and-tumble, free-for-all fighting.
      [PJC]

WordNet (r) 3.0 (2006):

in-fighting
    n 1: conflict between members of the same organization (usually
         concealed from outsiders)
    2: boxing at close quarters

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "infighting":
   Chinese boxing, aggressiveness, bellicosity, belligerence,
   bickering, bout, boxing, boxing match, close fighting,
   combativeness, contentiousness, dissension, dissent, dissidence,
   faction, factiousness, fighting, fisticuffs, flak, irascibility,
   irritability, litigiousness, partisan spirit, partisanship,
   prizefight, prizefighting, pugilism, pugnacity, quarrelsomeness,
   savate, shadowboxing, shrewishness, spar, the clinches, the fights,
   the ring