Geometric isomerism, labelled Z,E or cis–trans, is a form of stereoisomerism that arises from restricted rotation about a carbon–carbon double bond (C=C). Because the double bond cannot rotate freely, the groups on each carbon are locked on one side or the other, giving distinct cis and trans (or Z and E) forms with different physical properties. It matters in fats, polymers and vision; for example, the cis and trans forms of but-2-ene cannot interconvert without breaking the π bond.

几何异构(标记为 Z,E 或顺反)是一类立体异构,源于碳碳双键(C=C)旋转受限。由于双键不能自由旋转,每个碳上的基团被锁定在一侧或另一侧,从而形成性质不同的顺式和反式(或 Z 和 E)构型。它在脂肪、聚合物和视觉中都很重要;例如 2-丁烯的顺式与反式构型,不打断 π 键便无法相互转化。