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]