原子轨道在线性组合成分子轨道时（即两个波函数相加得到的分子轨道），能量较高的分子轨道叫反键轨道。反键轨道总是与成键轨道成对出现，其余为非键轨道。

反键轨道中，核间的电子的机率密度小。电子填入反键轨道中会使分子的稳定性降低。

In chemical bonding theory, an antibonding orbital is a type of molecular orbital that, if occupied by electrons, weakens the bond between two atoms and helps to raise the energy of the molecule relative to the separated atoms. Such an orbital has one or more nodes in the bonding region between the nuclei. The density of the electrons in the orbital is concentrated outside the bonding region and acts to pull one nucleus away from the other and tends to cause mutual repulsion between the two atoms.