Non-Boltzmann electron spin-state population produced in thermal or photochemical reactions, either from a combination of radical pairs (called radical-pair mechanism), or directly from the triplet state (called triplet mechanism) and detectable by
ESR spectroscopy.
Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 313 (https://doi.org/10.1351/pac200779030293)