tautomer
Chemistry term for one of two or more isomers that readily interconvert. Used in organic chemistry contexts.
Keto-enol tautomers are commonly seen in organic chemistry.
異性体 (isomer) is a broader term; 互変異性体 specifically refers to tautomers that interconvert.
Compound of 互変 (tautomerism) + 異性体 (isomer). The exact historical derivation is uncertain; the spelling is conventionally associated with the meaning.