The molecular orbital whose occupation by electrons decreases the total bonding (as usual, increases the total energy) of a molecule. The energy level of an antibonding MO
lies higher than the average of the valence atomic orbitals of the atoms constituting the molecule.
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1923 (https://doi.org/10.1351/pac199971101919)