Integral of the electric current over time. The smallest electric charge found on its own is the elementary charge, e, the charge of a proton.
Green Book, 2nd ed., p. 14 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 970 (https://doi.org/10.1351/pac199668040957)