from
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]