muon -
An unstable elementary particle in the lepton family, having similar properties to the electron but with a mass 209 times greater.
Wikipedia
The muon (from the Greek letter mu used to represent it) is an elementary particle similar to the electron, with a negative electric charge and a spin of ⁄2. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton. It is an unstable subatomic particle with the second longest mean lifetime, exceeded only by that of the free neutron.
OmegaWiki Dictionary
Ω
muon An elementary particle with negative electric charge and a spin of 1/2.