https://doi.org/10.1351/goldbook.C01092
Descriptors which show the relationship between two @[email protected] attached to separate atoms that are connected by a double bond or are contained in a ring. The two @[email protected] are said to be located cis to each other if they lie on the same side of a plane. If they are on opposite sides, their relative position is described as trans. The appropriate reference plane of a double bond is perpendicular to that of the relevant σ-bonds and passes through the double bond. For a ring (the ring being in a @[email protected], real or assumed, without re-entrant angles at the two substituted atoms) it is the mean plane of the ring(s). For @[email protected] the terms cis and trans may be ambiguous and have therefore largely been replaced by the @[email protected] convention for the nomenclature of organic compounds. If there are more than two entities attached to the ring the use of cis and trans requires the definition of a reference substituent.
Sources:
Blue Book Guide, p. 149 [Terms] [Book]
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2203 [Terms] [Paper]