Onium

The suffix -onium (plural: onia) refers to the bound state of a particle and its antiparticle. They are usually named by adding the suffix -onium to the name of the constituting particle. However, despite its name, muonium is not a muon–antimuon onium because IUPAC assigned the name to the system of an antimuon bound with an electron. (Muon–antimuon onium would be named muononium.)