Energy required to detach an electron from the singly charged negative ion (energy for the process
E01977-1.png). The equivalent more common definition is the energy released (E initial- E final) when an additional electron is attached to a neutral atom or molecule.
Sources:
Green Book, 2nd ed., p. 20 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
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 1546 (https://doi.org/10.1351/pac199163101541)
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1109 (https://doi.org/10.1351/pac199466051077)