Meaning

noun an elementary particle with a negative charge and a half-life of 2 microsecond; decays to electron and neutrino and antineutrino

originContraction of the earlier term mu-meson; the particle has now been recategorised as a lepton. Coined by Italian physicist Enrico Fermi in 1951 in his book Elementary Particles.

Etymology from Wiktionary, CC BY-SA 3.0