The molecular orbital whose occupation by electrons increases the total bonding (usually, lowers the total energy) of a molecule. Generally, the energy level of a bonding MO
lies lower than the average of the valence orbitals of the atoms constituting the molecule.
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1929 (https://doi.org/10.1351/pac199971101919)