Stereoisomeric olefins 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.
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2204 (https://doi.org/10.1351/pac199668122193)