A measure of the probability that a given ionization process will occur when an atom or molecule interacts with an electron or a photon.
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)