An onium (plural: onia) is a bound state of a particle and its antiparticle. They are usually named by adding the suffix -onium to the name of the constituting particle except for Muonium which, despite its name, not a bound muon–antimuon onium, but an electron–antimuon bound state, and whose name was assigned by IUPAC. A muon–antimuon onium would be named true muonium or muononium.