An electronic state that arises by the excitation of a valence electron to a diffuse atomic-like orbital. For instance, the lowest lying Rydberg state (n, 3S) of NH3
can be described as resulting from the excitation of a non-bonding orbital to a 3S Rydberg orbital.
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1961 (https://doi.org/10.1351/pac199971101919)