noun
: 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: 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