阻转异构现象是某些分子单键自由旋转受阻时，产生光活性异构体的现象，也可看作是构象异构的一类。其英文名为Atropisomer，来源于希腊文，其中a意为“不”，tropos意为“旋转”。该名称首先由Kuhn于1933年提出。

阻转异构体也称位阻光活性异构体，是一类含有手性轴的旋光异构体。与大部分含不对称原子所形成的手性化合物不同的是，含手性轴的旋光异构体不一定非要化学条件才可以互相转化，它们的分子受热具有一定能量后，便有可能形成化学平衡。相比较于构象异构体，阻转异构体必须相对稳定。根据Oki的定义，阻转异构体在某温度下互相转变的半衰期应当大于1000s。

Atropisomers are stereoisomers arising because of hindered rotation about a single bond, where energy differences due to steric strain or other contributors create a barrier to rotation that is high enough to allow for isolation of individual conformers. The word atropisomer (Gr., άτροπος, atropos, meaning "without turn") was coined in application to a theoretical concept by German biochemist Richard Kuhn for Karl Freudenberg's seminal Stereochemie volume in 1933. Atropisomerism was first experimentally detected in a tetra substituted biphenyl, a diacid, by George Christie and James Kenner in 1922. Michinori Ōki further refined the definition of atropisomers taking into account the temperature-dependence associated with the interconversion of conformers, specifying that atropisomers interconvert with a half-life of at least 1000 seconds at a given temperature, corresponding to an energy barrier of 93 kJ mol (22 kcal mol ) at 300 K (27 °C).