The Collaborative International Dictionary of English v.0.48:

Annihilate \An*ni"hi*late\, v. t. [imp. & p. p. Annihilated;
   p. pr. & vb. n. Annihilating.] [L. annihilare; ad +
   nihilum, nihil, nothing, ne hilum (filum) not a thread,
   nothing at all. Cf. File, a row.]
   1. To reduce to nothing or nonexistence; to destroy the
      existence of; to cause to cease to be.
      [1913 Webster]

            It impossible for any body to be utterly
            annihilated.                          --Bacon.
      [1913 Webster]

   2. To destroy the form or peculiar distinctive properties of,
      so that the specific thing no longer exists; as, to
      annihilate a forest by cutting down the trees. "To
      annihilate the army." --Macaulay.
      [1913 Webster]

   3. To destroy or eradicate, as a property or attribute of a
      thing; to make of no effect; to destroy the force, etc.,
      of; as, to annihilate an argument, law, rights, goodness.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Annihilate \An*ni"hi*late\ (an*n[imac]"h[i^]*l[asl]t), a.
   Annihilated. [Archaic] --Swift.
   [1913 Webster]

WordNet (r) 3.0 (2006):

annihilate
    v 1: kill in large numbers; "the plague wiped out an entire
         population" [syn: eliminate, annihilate, extinguish,
         eradicate, wipe out, decimate, carry off]

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "annihilate":
   abate, abolish, abrogate, abscind, amputate, annul, ban, bar,
   bereave of life, blot out, bob, bring to naught, cancel,
   carry away, carry off, chloroform, clip, crop, cull, cut, cut away,
   cut down, cut off, cut out, decimate, demolish, deprive of life,
   deracinate, destroy, dispatch, dispose of, do away with, do for,
   do to death, dock, efface, eliminate, end, enucleate, eradicate,
   erase, except, excise, exclude, execute, expunge, exterminate,
   extinguish, extirpate, finish, finish off, immolate, invalidate,
   isolate, kill, knock off, launch into eternity, liquidate, lop,
   lynch, make away with, martyr, martyrize, massacre, murder,
   mutilate, negate, negative, nip, nullify, obliterate, pare, peel,
   pick out, poison, prune, purge, put away, put down, put to death,
   put to sleep, quash, quell, quench, raze, remove, remove from life,
   repeal, revoke, root out, root up, rout, ruin, rule out, sacrifice,
   set apart, set aside, shave, shear, slay, squash, stamp out,
   starve, strike off, strip, strip off, suppress, sweep away,
   take life, take off, take out, truncate, unbuild, undo, uproot,
   vitiate, void, wipe out, wrack, wreck