The Collaborative International Dictionary of English v.0.48:

Annihilation \An*ni`hi*la"tion\, n. [Cf. F. annihilation.]
   1. The act of reducing to nothing, or nonexistence; or the
      act of destroying the form or combination of parts under
      which a thing exists, so that the name can no longer be
      applied to it; as, the annihilation of a corporation.
      [1913 Webster]

   2. The state of being annihilated. --Hooker.
      [1913 Webster]

WordNet (r) 3.0 (2006):

annihilation
    n 1: destruction by annihilating something [syn: annihilation,
         obliteration]
    2: total destruction; "bomb tests resulted in the annihilation
       of the atoll" [syn: annihilation, disintegration]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "annihilation":
   abolishment, abolition, abscission, amputation, annulment, bane,
   biological death, cessation of life, choking, choking off,
   clinical death, crossing the bar, curtains, death, death knell,
   debt of nature, decease, demise, departure, deracination,
   destruction, dissolution, doom, dying, ebb of life, elimination,
   end, end of life, ending, eradication, eternal rest, excision,
   exclusion, exit, expiration, extermination, extinction,
   extinguishment, extirpation, final summons, finger of death, going,
   going off, grave, hand of death, jaws of death, knell, last debt,
   last muster, last rest, last roundup, last sleep, leaving life,
   liquidation, loss of life, making an end, mutilation, negation,
   nullification, parting, passing, passing away, passing over,
   perishing, purge, quietus, release, rescission, rest, reward,
   rooting out, sentence of death, shades of death, shadow of death,
   silencing, sleep, snuffing out, somatic death, stifling,
   strangulation, suffocation, summons of death, suppression,
   uprooting, voiding