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cis is a Latin word meaning "on the same side" and is the opposite of trans. See Ciskei and Transkei, also Cisalpine Gaul and Transalpine Gaul.

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 Geometric isomerism for more on this.

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 mathematics, eix = cos x + i sin x (see Euler's formula). Cis is a now seldom used abbreviation for "cosine plus i sine" (of x). Usually eix or exp(ix) is best used.