Tautomerism: A reaction which involves simple proton transfer in an intramolecular fashion is called a tautomerism. It is one type of kinetic functional isomerism. It is a phenomenon by which two constitutional isomers become mutually inter-convertible & exist in dynamic equilibrium. The two different structures are called tautomers.
Examples: Propanone and propane-2-ol are tautomers of each other.