In cis-trans nomenclature, what does 'cis' represent?

In cis-trans nomenclature, 'cis' refers to a situation where two substituents or groups are located on the same side of a double bond or a ring. This designation is particularly useful in describing geometric isomers of alkenes or cyclic compounds. The term 'cis' originates from Latin, meaning "on this side of," which aligns with the chemical concept of proximity.

In this context, 'cis' corresponds to the Z notation in the Cahn-Ingold-Prelog priority system. When assigning the Z configuration, the groups of higher priority on either end of the double bond are on the same side, which directly relates to the definition of 'cis.' Thus, choosing this option correctly identifies the relationship described in the question.

On the other hand, 'E' generally indicates opposite sides of the double bond, while 'R' and 'S' pertain to the configuration of chiral centers, not to the geometric arrangement of groups around multiple bonds or rings. Therefore, understanding this specific context of geometric isomerism clarifies why 'cis' is represented by the Z configuration.