Wiktionary
protonium
n. (context physics English) An exotic atom consisting of a proton bound to an antiproton
Wikipedia
Protonium
Protonium (symbol: Pn), also known as antiprotonic hydrogen, is a type of exotic atom in which a proton (symbol: p) and an antiproton (symbol: ) orbit each other. Since protonium is a bound system of a particle and its corresponding antiparticle, it is one of many types of type of onium.
Like all onia, protonium is a boson with all quantum numbers ( baryon number, flavour quantum numbers, etc.) and electrical charge equal to 0.