tautomer Syllabification: (tau·to·mer)Pronunciation: /ˈtôtəmər/Definition of tautomer noun Chemistry each of two or more isomers of a compound that exist together in equilibrium, and are readily interchanged by migration of an atom or group within the molecule. Derivatives tautomericPronunciation: /ˌtôtəˈmerik/ adjective tautomerismPronunciation: /tôˈtäməˌrizəm/ noun Origin: early 20th century: blend of tauto- 'same' and isomertautomer in other Oxford dictionaries Definition of tautomer in the British & World English dictionary