Particle Physics an unstable, negatively charged lepton with a mass of 105.7 MeV, c. 207 times that of an electron, and a mean lifetime of 2.2×10 second: it decays into an electron, a neutrino, and an antineutrino
An elementary particle in the lepton family (not a meson), having a mass 209 times that of the electron, a negative electric charge, and a mean lifetime of 2.2 × 10-6 seconds. Also called mu meson. See Table at subatomic particle.