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.