Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Dimorphism is the condition of the appearance of the
            same species under two dissimilar forms. --Darwin.

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

Source : WordNet®

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)