The preferential formation in a chemical reaction of one stereoisomer over another.
When the stereoisomers are enantiomers, the phenomenon is called enantioselectivity and is quantitatively expressed by the
enantiomer excess;
when they are diastereoisomers, it is called diastereoselectivity and is quantitatively expressed by the diastereoisomer excess.
Source:
PAC, 1996, 68, 2193
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
on page 2219
See also:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1167