nonbonding molecular orbital

https://dx.doi.org/10.1351/goldbook.NT07083
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