A form of isomerism associated with compounds
containing a double bond. Like groups in such compounds may be either on
the same side of the plane of the double bond (cis-form) or on opposite
sides (trans-form). E.g., maleic acid and fumaric
acid are, respectively, cis- and trans-forms. See also stereoisomerism.
cis-1,2-dichloroethene (left); trans-1,2-dichloroethene