This article is about uses of the word "cis"; for the abbreviation "CIS" see CIS.

Cis is a Latin word meaning "on the same side" and is the opposite of trans. Its uses include:

• In chemistry, a double bond in which the greater radical on both ends is on the same side of the bond is called cis. Compare with trans. In the example shown on the right, both hydrogen molecules, (and both fluorine molecules) are on the same side of the carbon chain. As the carbons are joined by a double bond, these cannot rotate around the molecule. See also Geometric isomerism.

• In genetics, cis- is a prefix used in terms such as "cis-regulation" to signify the co-location of two or more genes on the same chromosome of a homologous pair.

• In geography, there are several pairs of places beginning Cis and Trans, including Ciskei and Transkei in South Africa, and Cisalpine Gaul and Transalpine Gaul.

• In mathematics, cis x is a now rarely used abbreviation for cos x + i sin x, which is equal to e^ix (see Euler's formula).