Elementary particle with the mass of an electron, but positively charged. It is the "anti-electron" generated during pair creation and emitted during beta plus decay.
Winfried Koelzer, Nuclear Terms, Karlsruher Institut for Technologie KIT (Ed.), ISBN German print version 2001: 3-923704-32-1, Translation by Informationskreis KernEnergie (Berlin), Karlsruhe, Germany 2013. https://www.euronuclear.org/info/encyclopedia/pdf/ Nuclear_Glossary-%202013-02-13.pdf, (Nucl Sfty - Amer) {1011}