An exotic atom, mu or (μ⁺e^-), formed when a positively charged muon (μ⁺) and an electron are bound by their mutual electrical attraction. It is a light, unstable isotope of hydrogen, with a muon replacing the proton. Muonium has a mass 0.11 times that of a hydrogen atom due to the lighter mass of the muon, and a mean lifetime of 2.2 microseconds, determined by the spontaneous decay of the muon (μ⁺ → e⁺ν_eν_μ).