tauto-

The same.

Greek tauto, contraction of to auto, the same.

A tautology occurs when the same thing is said twice in different words; a tautomer (Greek meros, a share) is each of two or more isomers of a compound which exist together in equilibrium; a tautonym is a taxonomic name in which the same word is used for both the genus and species elements, for example Coccothraustes coccothraustes, the hawfinch.