Mu (letter)

Mu (uppercase/lowercase Μ μ) is a letter of the Greek alphabet used to represent the "m" sound in Ancient and Modern Greek. In the system of Greek numerals, it has a value of 40. Letters that came from it include the Roman M and Cyrillic М.
In mathematics and statistics, the lowercase μ is used to represent population mean and the Möbius function. It is also used to represent coefficient of friction, micrometre and muon in physics.