Elementary particle not affected by the strong force having a spin quantum number 1/2, a negative elementary charge and a rest mass of 0.000548579903(13) u.
Source:
Green Book, 2nd ed., p. 93 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also:
PAC, 1982, 54, 1533. 'Glossary of terms used in nuclear analytical chemistry (Provisional)' on page 1540 (https://doi.org/10.1351/pac198254081533)