Normally in the nomenclature of particle physics, an atom composed of a positively charged particle bound to an electron is named after the positive particle with "-ium" replacing an "-on" suffix, in this case "muium". Replacing "-on" with (or otherwise appending) "-onium" is mostly used for bound states of a particle with its own antiparticle. The exotic atom consisting of a muon and an antimuon (which is yet to be observed) is known as true muonium.