https://doi.org/10.1351/goldbook.S05991
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 enantiomeric excess; when they are diastereoisomers, it is called 'diastereoselectivity' and is quantitatively expressed by the diastereoisomer excess.