The Collaborative International Dictionary of English v.0.48:

Decapitate \De*cap"i*tate\, v. t. [imp. & p. p. Decapitated;
   p. pr. & vb. n. Decapitating.] [LL. decapitatus, p. p. of
   decapitare; L. de- + caput head. See Chief.]
   1. To cut off the head of; to behead.
      [1913 Webster]

   2. To remove summarily from office. [Colloq. U. S.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

decapitate
    v 1: cut the head of; "the French King was beheaded during the
         Revolution" [syn: decapitate, behead, decollate]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "decapitate":
   behead, bowstring, burn, crucify, decimate, decollate,
   defenestrate, demolish, electrocute, execute, garrote, guillotine,
   head, inflict capital punishment, lapidate, neck, put to death,
   raze, ruin, shoot, stone, strangle, unbuild, undo, unmake, wrack,
   wreck