左旋与右旋是指有机化合物的对映体对偏振光中分别使光向反时针或顺时针方向旋转。会令偏振光左旋或右旋的异构体会被称为左旋体和右旋体。在有机化学中，通常用(+)表示右旋，(-)表示左旋。如L-构型的右旋酒石酸，其全名可写作L(+)-酒石酸。

Dextrorotation and levorotation (also spelled as laevorotation) are terms describing circular direction of rotating plane-polarized light. If the light rotates clockwise as it approaches an observer, this is known as dextrorotation (rotation to the right); if it rotates counterclockwise, then it exhibits levorotation (rotation to the left).