A bonding molecular orbital is a molecular orbital of lower energy than the atomic orbitals from which it forms, with increased electron density between the nuclei. By concentrating electron density between the nuclei, it holds them together and stabilises the molecule. It is formed by in-phase overlap, in contrast to the antibonding orbital formed by out-of-phase overlap.

成键分子轨道是能量低于其所由形成的原子轨道、且两核之间电子密度增大的分子轨道。由于把电子密度集中于两核之间,它使两核结合并稳定分子。它由同相重叠形成;与之相对的是由反相重叠形成的反键轨道。