The negative electric charge which appears on the electron or univalent ions [ 1.602 x 10E-19 coulombs or 4.803 x 10E-10 electrostatic units (esu)].
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2186 (https://doi.org/10.1351/pac199062112167)