同分异构体(isomer)，亦称异构体。分子式相同但构型不同的两种或两种以上的物质，其差别只在於组成原子的排列不同。通常指立体异构体（而不是同分异构体现象或互变异构体现象），其中还分为两类：旋光异构体，亦称对映异构体（参阅optical activity），以镜像成对地产生；几何异构体往往因分子结构的刚性而形成，在有机化合物中，这种刚性通常是由双键（参阅bonding）或环状结构引起的。在两个碳原子之间为双键的情况中，如果每个碳原子还键合着两个其他的基团，而且都严格固定在同一平面上，那麽相应的基团可以在C＝C键的同一侧（顺式）或彼此分布於C＝C键两侧（反式）。对於都在同一平面上的环状结构也可以作类似的区分，即组分基团在同一侧的同分异构体和组分基团在平面两侧的同分异构体。不是对映异构体的非对映异构体也属於这一类。大多数的顺-反式同分异构体都是有机化合物。

English version：

isomer

One of two or more substances with identical molecular formulas but different configurations, differing only in the arrangement of their component atoms. It usually refers to stereoisomers (rather than constitutional isomers or tautomers; see isomerism, tautomerism), of which there are two types. Optical isomers, or enantiomers (see optical activity), occur in mirror-image pairs. Geometric isomers are often the result of rigidity in the molecular structure; in organic compounds, this is usually due to a double bond (see bonding) or a ring structure. In the case of a double bond between two carbon atoms, if each has two other groups bonded to it and all are rigidly in the same plane, the corresponding groups can be on the same side (cis) of the C&doublehorzbond;C bond or across the C&doublehorzbond;C bond (trans) from each other. An analogous distinction can be made for ring structures that are all in a plane, between isomers whose substituent groups are on the same side and isomers whose substituent groups are on both sides of the plane. Diastereomers that are not enantiomers also fall into this category. Most cis-trans isomers are organic compounds.