Diastereomers are stereoisomers that are not mirror images of one another. They matter because, unlike enantiomers, they differ in ordinary physical properties such as melting point and solubility and so can be separated by conventional means. They typically arise when a molecule has more than one stereocentre; for example, among the stereoisomers of 2,3-dibromobutane, the meso form is a diastereomer of the enantiomeric pair.

非对映异构体是彼此不为镜像关系的立体异构体。它们之所以重要,是因为与对映异构体不同,它们在熔点、溶解度等普通物理性质上有差别,因而可用常规方法分离。它们通常出现在分子含有一个以上立体中心时;例如,在 2,3-二溴丁烷的各立体异构体中,内消旋体即为那一对对映体的非对映异构体。