The muon is the second lightest lepton. It has an electric charge of -1. It is identical to the electron but is 207 times more massive. It decays with a mean lifetime of 2.2 × 10-6 second. Muons occur commonly in cosmic rays. Despite its name, a muon (short for "mu meson") is no longer classified as a meson.
On learning of the discovery of the muon, the physicist Isidor Isaac Rabi said: "Who ordered that?"
Muonium is a short-lived entity formed by a muon and its antiparticle attracted together and revolving about a common center.