The Collaborative International Dictionary of English v.0.48:

Heterogeneous \Het`er*o*ge"ne*ous\, a. [Gr. ?; ? + ? race, kind;
   akin to E. kin: cf. F. h['e]t['e]rog[`e]ne.]
   Differing in kind; having unlike qualities; possessed of
   different characteristics; dissimilar; -- opposed to
   homogeneous, and said of two or more connected objects, or of
   a conglomerate mass, considered in respect to the parts of
   which it is made up. -- Het`er*o*ge"ne*ous*ly, adv. --
   Het`er*o*ge"ne*ous*ness, n.
   [1913 Webster]

   Heterogeneous nouns (Gram.), nouns having different genders
      in the singular and plural numbers; as, hic locus, of the
      masculine gender in the singular, and hi loci and h[ae]c
      loca, both masculine and neuter in the plural; hoc
      c[ae]lum, neuter in the singular; hi c[ae]li, masculine in
      the plural.

   Heterogeneous quantities (Math.), such quantities as are
      incapable of being compared together in respect to
      magnitude, and surfaces and solids.

   Heterogeneous surds (Math.), surds having different radical
      signs.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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