The Collaborative International Dictionary of English v.0.48:

Isomorphism \I`so*mor"phism\, n. [Cf. F. isomorphisme.]
   1. (Crystallog.) A similarity of crystalline form between
      substances of similar composition, as between the
      sulphates of barium (BaSO4) and strontium (SrSO4). It
      is sometimes extended to include similarity of form
      between substances of unlike composition, which is more
      properly called hom[oe]omorphism.

   2. (Math.) A one-to-one mapping of one set onto another set
      which preserves the relations between the elements of the
      domains of the sets.
      [PJC]