electron paramagnetic resonance (EPR)

https://doi.org/10.1351/goldbook.E02005
Electron paramagnetic resonance (EPR) and/or electron spin resonance (ESR) is defined as the form of spectroscopy concerned with microwave-induced transitions between magnetic energy levels of electrons having a net spin and orbital angular momentum. The term electron paramagnetic resonance and the symbol EPR are preferred and should be used for primary indexing.
Source:
PAC, 1989, 61, 2195. 'Recommendation for EPR/ESR nomenclature and conventions for presenting experimental data in publications (Recommendations 1989)' on page 2196 (https://doi.org/10.1351/pac198961122195)