natural bond orbital (NBO)

The orbital which is formed from natural hybrid orbitals. For a localized σ-bond between atoms A and B, the NBO is:

$σ AB = c A h A + c B h B$

where $h A$ and $h B$ are the natural hybrids centred on atoms A and B. NBOs closely correspond to the picture of localized bonds and lone pairs as basic units of molecular structure, so that is possible to conveniently interpret ab initio wave-functions in terms of the classical Lewis structure concepts by transforming these functions to NBO form.

Source:

PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1954