This is defined by analogy with
enantiomer excess, as

[and the percent diastereoisomer excess as

], where the
mole fractions of two
diastereoisomers in a mixture or the fractional yields of two
diastereoisomers formed in a reaction are

and

(

)
The term is not applicable if more than two
diastereoisomers are present. Frequently this term is abbreviated to
Source:
PAC, 1996, 68, 2193
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
on page 2205
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
https://doi.org/10.1351/goldbook.