A molecular orbital whose occupation by electrons does not contribute (or contributes insignificantly) to the binding energy of the molecule. Generally, a nonbonding
MO represents the highest occupied molecular orbital of a molecule.
Source:
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1955 (https://doi.org/10.1351/pac199971101919)