"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.
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