"pertaining to or characterized by isomerism," 1831, from German isomerisch (Berzelius, 1831, in a paper on the "Composition of the Tartaric and Paratartaric Racemic Acids"), from Greek isomeres "sharing equality, having equal parts or shares," from iso- "equal" (see iso-) + meros "part, share" (from PIE root *(s)mer- (2) "to get a share of something"). Isomerous is from 1845 in botany, 1840 in chemistry.

"the same in form, alike," 1862 [Robert Gordon Latham, "Elements of Comparative Philology"], from iso- "equal, identical" + -morphic, from Greek morphē "form, shape," a word of uncertain etymology. Earlier adjective was isomorphous (1821).

"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]

"equality before the law," c. 1600, from Italian or Latin, ultimately from Greek isonomia "equality of rights, the equality of a Greek democracy," from isos "equal, identical" (see iso-) + nomos "law" (see -nomy). Related: Isonomic (1851), which appears to be a separate formation in geology. Greek also had isoteleia in reference to an equality before the law sometimes granted to aliens in Athens, "equality of tax and tribute."