E, Z

The approved stereodescriptors of stereoisomeric alkenes R1R2C=CR3R4 ( R1R2 , R3R4 ); neither R1 nor R2 need be different from R3 or R4 ), cumulenes R1R2C(=C=C)n=CR3R4 and related systems e.g. R1R2C=NOH , HON=C{[CH2]n}2C=NOH . The group of highest CIP priority attached to one of the terminal doubly bonded atoms of the alkene, oxime, etc. or cumulene (i.e. R1 or R2 ) is compared with the group of highest precedence attached to the other (i.e. R3 or R4 ). The stereoisomer is designated as Z (zusammen = together) if the groups lie on the same side of a reference plane passing through the double bond and perpendicular to the plane containing the bonds linking the groups to the double-bonded atoms; the other stereoisomer is designated as E (entgegen = opposite). The descriptors may be applied to structures with a fractional bond order between one and two; and to double bonds involving elements other than carbon. They are not used to describe ring substitution relationships.
See also: cis-trans isomers
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2206 (https://doi.org/10.1351/pac199668122193)