The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster] Tautoousian