The orbital which is formed from natural hybrid orbitals. For a localized σ-bond between atoms A and B, the NBO is: \[\sigma _{\text{AB}}=c_{\text{A}}\ \text{h}_{\text{A}}+c_{\text{B}}\ \mathrm{h}_{\text{B}}\] where \(\text{h}_{\text{A}}\) and \(\text{h}_{\text{B}}\) 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.
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1954 [Terms] [Paper]