The Collaborative International Dictionary of English v.0.48:

Bandit \Ban"dit\ (b[a^]n"d[i^]t), n.; pl. Bandits
   (b[a^]n"d[i^]ts), or Banditti (b[a^]n*d[i^]t"t[i^]). [It.
   bandito outlaw, p. p. of bandire to proclaim, to banish, to
   proscribe, LL. bandire, bannire. See Ban an edict, and cf.
   Banish.]
   An outlaw; a brigand.
   [1913 Webster]

         No savage fierce, bandit, or mountaineer. --Milton.
   [1913 Webster]

   Note: The plural banditti was formerly used as a collective
         noun.
         [1913 Webster]

               Deerstealers are ever a desperate banditti. --Sir
                                                  W. Scott.
         [1913 Webster]

WordNet (r) 3.0 (2006):

bandit
    n 1: an armed thief who is (usually) a member of a band [syn:
         bandit, brigand]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "bandit":
   air armada, air force, badman, bogey, bravo, brigand, bummer,
   combat plane, cutthroat, dacoit, desperado, enemy aircraft,
   footpad, forager, freebooter, gangster, highwayman, holdup man,
   hoodlum, looter, mobsman, mobster, pillager, plunderer, racketeer,
   raider, ravager, sacker, slaunchways, thug, villain



Bouvier's Law Dictionary, Revised 6th Ed (1856):

BANDIT. A man outlawed; one who is said to be under ban.