Onium
Onium
Quantum state of a particle and its antiparticle
An onium (plural: onia) is a bound state of a particle and its antiparticle.[1] These states are usually named by adding the suffix -onium to the name of one of the constituent particles (replacing an -on suffix when present), with one exception for "muonium"; a muon–antimuon bound pair is called "true muonium" to avoid confusion with old nomenclature.[lower-alpha 1]