The Collaborative International Dictionary of English v.0.48:

Polymorph \Pol"y*morph\, n. [Gr. ? multiform; poly`s many +
   morfh` form: cf. F. polymorphe.] (Crystallog.)
   A substance capable of crystallizing in several distinct
   forms; also, any one of these forms. Cf. Allomorph.
   [1913 Webster]

WordNet (r) 3.0 (2006):

polymorph
    n 1: an organism that can assume more than one adult form as in
         the castes of ants or termites