Definition of cis–trans isomerism
: geometric isomerism in unsaturated compounds or cyclic compounds depending usually on the presence in the molecule of a pair of substituted groups (as unsymmetrically substituted methylene groups) so that the isomers have comparable substituents on either the same or opposite sides of the molecule: such as a : stereoisomerism in compounds (as maleic and fumaric acids) containing one or more carbon-to-carbon double bonds b : syn-anti isomerism c : stereoisomerism in various cyclic compounds (as disubstituted cyclohexanes) which in many cases may also exhibit optical isomerism
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