From AMS Glossary
(Or antielectron.) A positively charged subatomic particle with the same mass and charge magnitude as that of the electron.
The electron's antiparticle, the positron was first observed in 1932 in cosmic rays by Carl Anderson using a Wilson cloud chamber.
Boorse, H. A., and L. Motz 1966. The World of the Atom. Vol. II, . 1261–1267.
Anderson, C. D. 1961. Amer. J. Phys.. 29. 825–830.