The transfer of an electron to a molecular entity, resulting in a molecular entity of (algebraically) increased negative charge. It is not an attachment.
See also: oxidation (Def. 1), reduction
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 1547 (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)