A muon is an elementary particle similar to an electron, with an electric charge of -1 e and a spin of 1/2, but it is about 207 times more massive and unstable, decaying into electrons, neutrinos, and antineutrinos in about 2.2 microseconds. Muons are produced in large quantities in the upper atmosphere by cosmic rays and are used in various applications, including probing the internal structure of materials and archaeological sites through muon tomography.