All electrons emitted from the surface of a solid except back scattered primary electrons. In practice, electrons emitted from the surface of a solid under particle bombardment which have a kinetic energy of less than 50 eV.
Source:
PAC, 1983, 55, 2023. 'Nomenclature, symbols and units recommended for in situ microanalysis (Provisional)' on page 2026 (https://doi.org/10.1351/pac198355122023)