The Collaborative International Dictionary of English v.0.48:

Incongruity \In`con*gru"i*ty\, n.; pl. Incongruities. [Pref.
   in- not + congruity: cf. F. incongruit['e].]
   [1913 Webster]
   1. The quality or state of being incongruous; lack of
      congruity; unsuitableness; inconsistency; impropriety.
      [1913 Webster]

            The fathers make use of this acknowledgment of the
            incongruity of images to the Deity, from thence to
            prove the incongruity of the worship of them. --Bp.
                                                  Stillingfleet.
      [1913 Webster]

   2. Disagreement of parts; lack of symmetry or of harmony.
      [Obs.]
      [1913 Webster]

   3. That which is incongruous; lack of congruity.
      [1913 Webster]

WordNet (r) 3.0 (2006):

incongruity
    n 1: the quality of disagreeing; being unsuitable and
         inappropriate [syn: incongruity, incongruousness] [ant:
         congruence, congruity, congruousness]

Moby Thesaurus II by Grady Ward, 1.0:

133 Moby Thesaurus words for "incongruity":
   absurdity, absurdness, ambiguity, ambivalence, antilogy, antinomy,
   asymmetry, bizarreness, bizarrerie, contrariety, contrast,
   departure, deviation, difference, disaccord, disaccordance,
   disagreement, disconformity, discongruity, discordance,
   discrepancy, discreteness, disparity, disproportion,
   disproportionateness, dissent, dissimilarity, dissonance,
   distinction, distinctness, divergence, divergency, diversity,
   drollery, drollness, eccentricity, equivocality, far cry,
   funniness, futility, heresy, heterodoxy, heterogeneity, hilarity,
   humorousness, illogic, illogicality, illogicalness,
   impoliticalness, impoliticness, inaccordance, inadvisability,
   inappropriateness, inaptitude, incoherence, incommensurability,
   incompatibility, inconclusiveness, inconsistency, inconsonance,
   inconvenience, inconveniency, ineptitude, inequality, inexpedience,
   inexpediency, infelicity, inharmoniousness, inharmony,
   inopportuneness, invalidity, irrationality, irreconcilability,
   laughability, ludicrousness, mixture, noncompliance,
   nonconcurrence, nonconformability, nonconformance, nonconformism,
   nonconformity, nonobservance, odds, opposition, originality,
   otherness, oxymoron, paradox, pricelessness, protest, quaintness,
   quizzicalness, reasonlessness, recalcitrance, recusance, recusancy,
   refractoriness, richness, ridiculousness, self-contradiction,
   senselessness, separateness, the funny side, unconformability,
   unconformity, undesirability, unfitness, unfittingness,
   unfortunateness, unlikeness, unmeetness, unorthodoxy,
   unprofitability, unprofitableness, unreasonableness,
   unscientificness, unseasonableness, unseemliness, unsoundness,
   unsuitability, untenableness, untimeliness, unwiseness,
   uselessness, variance, variation, variegation, variety,
   whimsicalness, wittiness, worthlessness, wrongness