The Collaborative International Dictionary of English v.0.48:

Absurdity \Ab*surd"i*ty\ (-[i^]*t[y^]), n.; pl. Absurdities
   (-t[i^]z). [L. absurditas: cf. F. absurdite.]
   1. The quality of being absurd or inconsistent with obvious
      truth, reason, or sound judgment. "The absurdity of the
      actual idea of an infinite number." --Locke.
      [1913 Webster]

   2. That which is absurd; an absurd action; a logical
      contradiction.
      [1913 Webster]

            His travels were full of absurdities. --Johnson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

absurdity
    n 1: a message whose content is at variance with reason [syn:
         absurdity, absurdness, ridiculousness]
    2: a ludicrous folly; "the crowd laughed at the absurdity of the
       clown's behavior" [syn: absurdity, fatuity,
       fatuousness, silliness]

Moby Thesaurus II by Grady Ward, 1.0:

162 Moby Thesaurus words for "absurdity":
   absurdness, act of folly, aimlessness, amphigory, anticness,
   babble, babblement, balderdash, bibble-babble, bizarreness,
   bizarrerie, blabber, blather, blunder, bombast, bootlessness,
   claptrap, craziness, curiousness, daftness, deformity, dottiness,
   double-talk, drivel, drollery, drollness, drool, dumb trick,
   eccentricity, emptiness, error, fallacy, fantasticality,
   fantasticalness, fatuity, fecklessness, fiddle-faddle,
   fiddledeedee, flummery, folderol, folly, foolishness, freakishness,
   fruitlessness, fudge, funniness, fustian, futility, gabble,
   galimatias, gammon, gibber, gibberish, gibble-gabble, gobbledygook,
   grotesqueness, grotesquerie, hilarity, hocus-pocus, hollowness,
   hopelessness, humbug, humorousness, illogicality, impossibility,
   impossible, impossibleness, impotence, imprudence, inanity,
   inconceivability, incongruity, indiscretion, ineffectiveness,
   ineffectuality, inefficacy, insanity, irrationality, jabber,
   jargon, laughability, ludicrousness, malformation, meaninglessness,
   monstrosity, monstrousness, mumbo jumbo, narrishkeit, niaiserie,
   no chance, nonsense, nonsensicality, nugacity, nuttiness, oddity,
   otiosity, outlandishness, outrageousness, oxymoron,
   pack of nonsense, palaver, paradox, peculiarity, pointlessness,
   prate, prattle, preposterousness, pricelessness, profitlessness,
   purposelessness, quaintness, queerness, quizzicalness, rant,
   rat race, richness, ridiculousness, rigamarole, rigmarole,
   rodomontade, rubbish, self-contradiction, senselessness, silliness,
   singularity, skimble-skamble, sottise, strangeness,
   stuff and nonsense, stultiloquence, stupid thing, stupidity,
   teratism, the absurd, the funny side, the impossible, trash,
   triviality, trumpery, twaddle, twattle, twiddle-twaddle,
   unimaginability, unproductiveness, unprofitability,
   unprofitableness, unreasonableness, unthinkability, unwise step,
   valuelessness, vanity, vaporing, vicious circle, waffling,
   weirdness, what cannot be, what cannot happen, whimsicalness,
   wildness, witlessness, wittiness, worthlessness



The Devil's Dictionary (1881-1906):

ABSURDITY, n.  A statement or belief manifestly inconsistent with
one's own opinion.