This is defined by analogy with enantiomer excess, as D1 - D2 [and the percent diastereoisomer excess as 100(D1- D2)], where the mole fractions of two diastereoisomers in a mixture or the fractional yields of two diastereoisomers formed in a reaction are D1 and D2 (D1 + D2 = 1). The term is not applicable if more than two diastereoisomers are present. Frequently this term is abbreviated to d.e.
See: diastereoisomerism, stereoselectivity
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2205 (https://doi.org/10.1351/pac199668122193)