"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)..
isomer
isomeric
isometric
isometrics
isometry
isomorph
isomorphic
isomorphism
isonomia
isopod
isopolity
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