σ → σ* transition

https://doi.org/10.1351/goldbook.S05435
An electronic transition described approximately as promotion of an electron from a 'bonding' σ orbital to an 'antibonding' σ orbital designated as σ *. Such transitions generally involve high transition energies, and appear close to or mixed with Rydberg transitions.
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2279 (https://doi.org/10.1351/pac199668122223)