One of a pair of identical groups c attached to a double bond (as in
abC=Cc2) is described as
pro-E if, when it is arbitrarily assigned CIP priority over the other group c, the stereodescriptor of the molecule becomes
E. The other group c is then described as
pro-Z.Source:
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2214 (https://doi.org/10.1351/pac199668122193)