The Collaborative International Dictionary of English v.0.48:

Heterogeneity \Het`er*o*ge*ne"i*ty\, n. [Cf. F.
   h['e]t['e]rog['e]n['e]it['e].]
   The state of being heterogeneous; contrariety.
   [1913 Webster]

         The difference, indeed the heterogeneity, of the two
         may be felt.                             --Coleridge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heterogeneity
    n 1: the quality of being diverse and not comparable in kind
         [syn: heterogeneity, heterogeneousness] [ant:
         homogeneity, homogeneousness]