Stereoisomericolefins or cycloalkanes (or hetero-analogues) which differ in the positions of atoms (or groups) relative
to a reference plane: in the cis-isomer the atoms are on the same side, in the trans-isomer they are on opposite sides.
Source:
PAC, 1996, 68, 2193
(Basic terminology of stereochemistry (IUPAC Recommendations 1996))
on page 2204
