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