映射异构体（英语：Enantiomer），又称对掌异构物、光学异构物、镜像异构物或对映异构体，不能与彼此立体异构体镜像完全重叠。

互为镜像（mirror images）的分子。不对称碳（asymmetric carbon）和四种不同的原子或原子基团链接，不对称碳为手性中心。当有n个手性中心时，则最多有2的n次方立体异构物。

来源于希腊文，具有左手对右手那样镜像关系的一对物质。无论怎样摆布都不能使这些镜像成为同一物。有对称平面的物质不能是映射体，因为它和它的镜像是等同的。乳酸那样的分子映射体，除了与其他不对称分子的化学反应以及与偏振光作用外，具有完全相同的化学物质。映射体在结晶学中很重要，因为许多晶体是由单个分子的右手型和左手型交替排列的。对晶体的完整描述，就是要说明这些型体彼此间是如何混合的。两种光学活性的酒石酸，即所谓d-酒石酸和l-酒石酸就是一对映射体的实例。

In chemistry, an enantiomer (/ɪˈnæntiəmər, ɛ-, -tioʊ-/ ə-NAN-tee-ə-mər; fromGreekἐνάντιος (enantíos), meaning "opposite",andμέρος (méros), meaning "part"), also known as an optical isomer, is one of two stereoisomers that are mirror images of each other that are non-superposable (not identical), much as one's left and right hands are the same except for being reversed along one axis (the hands cannot be made to appear identical simply by reorientation). Organic compounds that contain a chiral carbon usually have two non-superposable structures. These two structures are mirror images of each other and are, thus, commonly called enantiomorphs (enantio = opposite ; morph = form), hence this structural property is now commonly referred to as enantiomerism.