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