Tautomerism

a special case of structural isomerism, often called dynamic isomerism. It is when a compound exists as a mixture of two forms, or isomers (in equilibrium). The two forms can each change to the other as when, for example, one isomer is extracted from the mixture, then some of the other isomer will change to re-establish equilibrium. The ability to make this reversible change is due to a mobile atom or group, often hydrogen, which alters its position in the molecule possibly with the changing of a double bond. Isomers produced in this way are called TAUTOMERS and each can give rise to stable derivatives.

Taken from Dictionary of Science