In chemistry, an enantiomer (/ɪˈnænti.əmər, ɛ-, -oʊ-/ ih-NAN-tee-ə-mər; from Ancient Greek ἐναντίος (enantíos) opposite, and μέρος (méros) part) – also called optical isomer, antipode, or optical antipode – is one of two stereoisomers that are nonsuperposable onto their own mirror image. Enantiomers are much like one's right and left hands; without mirroring one of them, hands cannot be superposed onto each other. No amount of reorientation in three spatial dimensions will allow the four unique groups on the chiral carbon (see chirality) to line up exactly. The number of stereoisomers a molecule has can be determined by the number of chiral carbons it has.
A molecule with chirality rotates plane-polarized light. A mixture of equal amounts of each enantiomer, a racemic mixture or a racemate, does not rotate light. 
Stereoisomers include both enantiomers and diastereomers. Diastereomers, like enantiomers, share the same molecular formula and are also nonsuperposable onto each other; however, they are not mirror images of each other.