A stereoisomer is an isomer that has the same atomic connectivity as another but differs in the spatial arrangement, or configuration, of its atoms. Stereoisomers matter because molecules can be built from identically bonded atoms yet still differ in three-dimensional shape, which affects their properties. They contrast with constitutional isomers, in which the connectivity itself differs; stereoisomers are further divided into enantiomers and diastereomers.

立体异构体是指与另一异构体原子连接方式相同、但原子的空间排列(即构型)不同的异构体。立体异构体之所以重要,是因为分子可以由连接方式完全相同的原子构成,却仍在三维形状上不同,从而影响其性质。它们与连接方式本身不同的构造异构体形成对照;立体异构体又进一步分为对映异构体和非对映异构体。