The Collaborative International Dictionary of English v.0.48:

Dissimilarity \Dis*sim`i*lar"i*ty\, n.
   Want of resemblance; unlikeness; dissimilitude; variety; as,
   the dissimilarity of human faces and forms. --Sir W. Jones.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dissimilarity
    n 1: the quality of being dissimilar [syn: dissimilarity,
         unsimilarity] [ant: similarity]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "dissimilarity":
   contrariety, contrast, departure, deviation, difference, disaccord,
   disaccordance, disagreement, disconformity, discongruity, discord,
   discordance, discrepancy, discreteness, disparity, dissemblance,
   dissent, dissimilitude, dissonance, distance, distinction,
   distinctness, divarication, divergence, divergency, diversity,
   far cry, heterogeneity, inaccordance, incompatibility, incongruity,
   inconsistency, inconsonance, inequality, inharmoniousness,
   inharmony, irreconcilability, margin, mixture, nonconformity, odds,
   offset, opposition, otherness, separateness, severance,
   unconformity, unlikeness, unorthodoxy, variance, variation,
   variegation, variety