In chemical nomenclature, a descriptor is a notational prefix placed before the systematic substance name, which describes the configuration or the stereochemistry of the molecule. Some of the listed descriptors should not be used in publications, as they no longer accurately correspond with the recommendations of the IUPAC. Stereodescriptors are often used in combination with locants to clearly identify a chemical structure unambiguously.

The descriptors, usually placed at the beginning of the systematic name, are not taken into account in the alphabetical sorting.

See: cis–trans isomerism

The descriptors cis (Latin, on this side of) and trans (Latin, over, beyond) are used in various contexts for the description of chemical configurations:

In organic structural chemistry, the configuration of a double bond can be described with cis and trans, in case it has a simple substitution pattern with only two residues. The position of two residues relative to one another at different points in a ring system or a larger molecule can also be described with cis and trans if the structure's configuration is rigid and does not allow simple inversion.

In inorganic complex chemistry, the descriptors cis and trans are used to characterize the positional isomers in octahedral complexes with A₂B₄X configuration or square planar complexes with A₂B₂X configuration.

The typographic presentation of cis and trans is italicised and in lower case letters.

The cis/trans nomenclature is not unambiguous for more highly substituted double bonds and is nowadays largely replaced by the (E)/(Z) nomenclature.