The Greek letter "mu" (μ) is the 12th letter of the Greek alphabet. In the Greek numeral system, it has a value of 40. "Mu" is often used in various scientific and mathematical contexts, representing concepts such as the coefficient of friction or the mean in statistics. In addition to its mathematical significance, "mu" is also used in physics to denote the permeability of a material in electromagnetism. The letter has a rich history in ancient Greek culture and is still widely recognized in modern language and symbolism, including its use in Greek mythology and philosophy.