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.
Source:
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1923 (https://doi.org/10.1351/pac199971101919)