The Collaborative International Dictionary of English v.0.48:

Massacre \Mas"sa*cre\, v. t. [imp. & p. p. Massacred; p. pr. &
   vb. n. Massacring.] [Cf. F. massacrer. See Massacre, n.]
   To kill in considerable numbers where much resistance can not
   be made; to kill with indiscriminate violence, without
   necessity, and contrary to the usages of nations; to butcher;
   to slaughter; -- limited to the killing of human beings.
   [1913 Webster]

         If James should be pleased to massacre them all, as
         Maximian had massacred the Theban legion. --Macaulay.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Massacre \Mas"sa*cre\, n. [F., fr. LL. mazacrium; cf. Prov. G.
   metzgern, metzgen, to kill cattle, G. metzger a butcher, and
   LG. matsken to cut, hew, OHG. meizan to cut, Goth.
   m['a]itan.]
   1. The killing of a considerable number of human beings under
      circumstances of atrocity or cruelty, or contrary to the
      usages of civilized people; as, the massacre on St.
      Bartholomew's Day; the St. Valentine's Day massacre; the
      Amritsar massacre; the Wounded Knee massacre.
      [1913 Webster +PJC]

   2. Murder. [Obs.] --Shak.
      [1913 Webster]

   Syn: Massacre, Butchery, Carnage.

   Usage: Massacre denotes the promiscuous slaughter of many who
          can not make resistance, or much resistance. Butchery
          refers to cold-blooded cruelty in the killing of men
          as if they were brute beasts. Carnage points to
          slaughter as producing the heaped-up bodies of the
          slain.
          [1913 Webster]

                I'll find a day to massacre them all,
                And raze their faction and their family. --Shak.
          [1913 Webster]

                If thou delight to view thy heinous deeds,
                Brhold this pattern of thy butcheries. --Shak.
          [1913 Webster]

                Such a scent I draw
                Of carnage, prey innumerable!     --Milton.
          [1913 Webster]

WordNet (r) 3.0 (2006):

massacre
    n 1: the savage and excessive killing of many people [syn:
         slaughter, massacre, mass murder, carnage,
         butchery]
    v 1: kill a large number of people indiscriminately; "The Hutus
         massacred the Tutsis in Rwanda" [syn: massacre,
         slaughter, mow down]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "massacre":
   annihilate, annihilating, annihilation, assassination, assault,
   attack, battering, bloodbath, bloodshed, bump off, butcher,
   butchering, butchery, carnage, commit carnage, commit genocide,
   commit mass murder, decimate, decimation, depopulate, destroy,
   disorderliness, eliminate, eradicate, execute, execution,
   exterminate, extermination, final solution, forcible seizure,
   genocide, holocaust, kill, killing, laying waste, liquidate,
   liquidation, looting, mass destruction, mass murder, mow down,
   murder, murder wholesale, murdering, obliterate, obstreperousness,
   onslaught, pillaging, pogrom, race extermination, race-murder,
   rape, riot, rioting, sacking, saturnalia of blood, slaughter,
   slaughtering, slay, slay en masse, slaying, sowing with salt,
   unruliness, violation, wholesale murder, wipe out