- a lepton similar in most respects to the electron except that it is unstable, it may be positively charged, and its mass is approximately 207 times greater; the positively charged muon is the antiparticle of the negatively charged muon. Symbol: μ
- a positive or negative elementary particle with a mass 207 times that of an electron and spin 1/2 . It was originally called the mu meson but is now classified as a lepton
