A racemic mixture, or racemate, is a 50:50 mixture of the two enantiomers of a chiral compound. It matters because, although each enantiomer alone rotates plane-polarised light, their equal and opposite rotations cancel, so the mixture shows no net optical rotation. A racemate is the usual outcome when a chiral product is formed from achiral starting materials without any chiral influence, for example via a flat carbenium ion attacked equally from either face.

外消旋混合物(外消旋体)是手性化合物两种对映体各占一半的混合物。它之所以重要,是因为尽管每一种对映体单独都能旋转平面偏振光,但二者大小相等、方向相反的旋光相互抵消,故混合物无净旋光性。当手性产物由非手性原料在没有任何手性影响下生成时(例如经由可从两面等同进攻的平面碳正离子),通常得到外消旋体。