The Collaborative International Dictionary of English v.0.48:

Dissimilitude \Dis`si*mil"i*tude\, n. [L. dissimilitudo, fr.
   dissimilis: cf. F. dissimilitude.]
   1. Want of resemblance; unlikeness; dissimilarity.
      [1913 Webster]

            Dissimilitude between the Divinity and images.
                                                  --Stillingfleet.
      [1913 Webster]

   2. (Rhet.) A comparison by contrast; a dissimile.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dissimilitude
    n 1: dissimilarity evidenced by an absence of likeness [syn:
         unlikeness, dissimilitude] [ant: alikeness,
         likeness, similitude]