An antibonding molecular orbital is a molecular orbital of higher energy than the atomic orbitals from which it forms, marked with an asterisk (*) and having a node between the nuclei. Because electron density is removed from between the nuclei, occupying it weakens the bond and destabilises the molecule. It is formed by out-of-phase overlap, in contrast to the bonding orbital.

反键分子轨道是能量高于其所由形成的原子轨道、标以星号(*)、且两核之间存在节面的分子轨道。由于电子密度被从两核之间移走,占据该轨道会削弱键并使分子不稳定。它由反相重叠形成;与成键轨道相对。