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.
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   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.
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                If thou delight to view thy heinous deeds,
                Brhold this pattern of thy butcheries. --Shak.
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                Such a scent I draw
                Of carnage, prey innumerable!     --Milton.
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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]