"that which has the same form as another but belongs to a different group," 1850 of mineral substances; 1885 in zoology, probably a back-formation from isomorphism (q.v.), but used earlier in German (1821)..
Entries linking to isomorph
isomorphism n.
"similarity of form," 1822, in John George Children's translation from French of Berzelius's "The Use of the Blow-pipe in Chemical Analysis," from French l'isomorphisme, from German Isomorphismus (1819), coined by German chemist Eilhard Mitscherlich (1794-1863) from Greek isos "equal, identical" (see iso-) + morphe "form, appearance," a word of uncertain etymology.
Mr. Children has, very properly in our estimation, wholly omitted the formulae, translating them into plain English in notes at the bottom of the page; we wish he had exerted the same discretionary judgment with respect to the isomorphisms and left them out likewise. [from a review of Children's book in The Quarterly Review of Science, Literature, and the Arts, vol. xiii, 1822]