Source : Webster's Revised Unabridged Dictionary (1913)

Isomorphism \I`so*mor"phism\, n. [Cf. F. isomorphisme.]
   (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.