Nonbonding orbitals are orbitals centered on atoms, but it doesn’t interact with other atomic orbitals. When a molecule is formed by the linear combination of atomic orbital (LCAO), the atomic orbitals that are left free are also called nonbonding molecular orbital. Term non-bonding is used simply because those orbitals are not bonding.

It is important to notice that these are orbitals that appear in the frontier region (higher energy or leading (frontier) orbitals) of a molecule. Hence the influence of these orbitals on the properties/reactivities of these molecules are high.

For instance, the nonbonding orbital on BH₃ renders Lewis acid features to the molecule.