The Collaborative International Dictionary of English v.0.48:

Pleomorphism \Ple`o*mor"phism\, n. [Gr. ? more + morfh` form.]
   1. (Crystallog.) The property of crystallizing under two or
      more distinct fundamental forms, including dimorphism and
      trimorphism.
      [1913 Webster]

   2. (Biol.) The theory that the various genera of bacteria are
      phases or variations of growth of a number of Protean
      species, each of which may exhibit, according to
      undetermined conditions, all or some of the forms
      characteristic of the different genera and species.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pleomorphism
    n 1: (chemistry) the existence of different kinds of crystal of
         the same chemical compound [syn: polymorphism,
         pleomorphism]
    2: (biology) the appearance of two or more distinctly different
       forms in the life cycle of some organisms