Takes place when an atom or molecule is ionized when it interacts with a solid surface. ionization occurs only when the work function of the surface, the temperature of the surface, and the ionization energy of the atom or molecule have an appropriate relationship.
Source:
PAC, 1991, 63, 1541. 'Recommendations for nomenclature and symbolism for mass spectroscopy (including an appendix of terms used in vacuum technology). (Recommendations 1991)' on page 1548 (https://doi.org/10.1351/pac199163101541)