In chemistry, an epimer is a stereoisomer of another compound that has a different configuration at only one of several stereogenic centers. Stereoisomers include enantiomers and diastereomers, both which contain a stereogenic center (excluding geometric isomers, which is a class of diastereomers).
For example, the sugars α-glucose and β-glucose are epimers. In α-glucose, the -OH group on the first (anomeric) carbon is in the direction opposite the methylene group. In β-glucose, the -OH group is oriented in the same direction as the methylene group . These two molecules are both epimers and anomers.
In this case β-D-glucopyranose and β-D-mannopyranose are epimers because the differ only in the stereochemistry at the 2 position. The hydroxyl group in β-D-glucopyranose is equatorial (in the "plane" of the ring) while in β-D-mannopyranose the 2 hydroxyl group is axial (up from the "plane" of the ring). These two molecules are epimers but not anomers.
In chemical nomenclature one of the epimeric pairs is given the prefix epi- for example in quinine and epi-quinine. When the pairs are enantiomers the prefix becomes ent-.