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A muon is a type (i.e., flavor) of lepton that has an electric charge of -1, like an electron, but has far more mass (about 200 times more) and is unstable, decaying with a lifetime of about 2 microseconds. They result from high-energy particle interactions, and decay into an electron and two neutrinos. Air showers can create them and muon sensors can be used to gather some kinds of data on them.
Muons have a role in confirming relativity: they are detected at Earth's surface, having been created high in Earth's atmosphere; this is taken as evidence of relativity's time-dilation effect, due to the relativistic speed of their downward travel. Their lifetime, demonstrable by laboratory experiment, is too short for them to travel that far unless it was lengthened by such time dilation.