What is "tautomerism"

Tautomerism \Tau*tom"er*ism\, n. [Gr. ?; ?, for ? ? the same + ? part.] (Chem.) The condition, quality, or relation of metameric substances, or their respective derivatives, which are more or less interchangeable, according as one form or the other is the more stable. It is a special case of metamerism; thus, the lactam and the lactim compounds exhibit tautomerism.

n. (context organic chemistry English) A form of isomerism in which a dynamic equilibrium between multiple isomers exists, such as that between an enol and a ketone.