The Collaborative International Dictionary of English v.0.48:

Dimorphism \Di*mor"phism\, n. [Cf. F. dimorphisme.]
   1. (Biol.) Difference of form between members of the same
      species, as when a plant has two kinds of flowers, both
      hermaphrodite (as in the partridge berry), or when there
      are two forms of one or both sexes of the same species of
      butterfly.
      [1913 Webster]

            Dimorphism is the condition of the appearance of the
            same species under two dissimilar forms. --Darwin.
      [1913 Webster]

   2. (Crystallog.) Crystallization in two independent forms of
      the same chemical compound, as of calcium carbonate as
      calcite and aragonite.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dimorphism
    n 1: (chemistry) the property of certain substances that enables
         them to exist in two distinct crystalline forms
    2: (biology) the existence of two forms of individual within the
       same animal species (independent of sex differences)