natural atomic orbital (NAO)

A valence-shell atomic orbital whose derivation involves diagonalising the localized block of the full density matrix of a given molecule associated with basis functions χ i A on that atom. A distinguishing feature of NAOs is that they meet the simultaneous requirement of orthonormality and maximum occupancy. For isolated atoms, NAOs coincide with natural orbitals. In a polyatomic molecule the NAOs (in contrast to natural orbitals that become delocalised over all nuclear centres) mostly retain one-centre character, and thus are optimal for describing the molecular electron density around each atomic centre.
PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1954