Cis isomer

Cis–trans isomerism, also known as geometric isomerism or configurational isomerism, is a term used in organic chemistry. The terms “cis” and “trans” are from Latin. Cis means that functional groups are on the same side of the carbon chain, and trans means that functional groups are on the opposite side of the carbon chain. It is used to describe molecules that are stereoisomers, that is, pairs of molecules which have the same formula but whose functional groups are rotated into a different orientation in three-dimensional space. It is not to be confused with E–Z isomerism, which is an absolute stereochemical description, and only to be used with alkenes. In general, stereoisomers contain double bonds that cannot rotate, or they may contain ring structures, where the rotation of bonds is restricted or prevented.Cis and trans isomers occur both in organic molecules and in inorganic coordination complexes. Cis and trans descriptors are not used for cases of conformational isomerism where the two geometric forms easily interconvert, such as most open-chain single-bonded structures; instead, the terms “syn” and “anti” would be used.

The term "geometric isomerism" is considered an obsolete synonym of "cis–trans isomerism" by IUPAC.

When the substituent groups are oriented in the same direction, the diastereomer is referred to as cis, whereas, when the substituents are oriented in opposing directions, the diastereomer is referred to as trans. An example of a small hydrocarbon displaying cis–trans isomerism is but-2-ene.

Alicyclic compounds can also display cis–trans isomerism. As an example of a geometric isomer due to a ring structure, consider 1,2-dichlorocyclohexane:

...
Wikipedia