An elementary particle with a negative charge and a half-life of 2 microsecond; decays to electron and neutrino and antineutrino; SYN. negative muon, mu-meson.
The second flavour of charged leptons (in order of increasing mass), with electric charge -1.
An elementary particle similar to the electron except for its mass which is 207 times greater than that of the electron. It has a half-life of 2 millionths of a second, decaying into electrons and neutrinos. The muon was originally thought to be a meson and is thus sometimes called a mu meson, although current opinion is that it is a lepton.