SOMO

https://doi.org/10.1351/goldbook.S05765
A Singly Occupied @M03996@ (such as the half-filled @H02838@ of a @R05066@).
See also:
frontier orbitals
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1165 [Terms] [Paper]