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.
Source:
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1929 (https://doi.org/10.1351/pac199971101919)