Mu (letter)

Greek alphabet
Α α Alpha	Β β Beta
Γ γ Gamma	Δ δ Delta
Ε ε Epsilon	Ζ ζ Zeta
Η η Eta	Θ θ Theta
Ι ι Iota	Κ κ Kappa
Λ λ Lambda	Μ μ Mu
Ν ν Nu	Ξ ξ Xi
Ο ο Omicron	Π π Pi
Ρ ρ Rho	Σ σ Sigma
Τ τ Tau	Υ υ Upsilon
Φ φ Phi	Χ χ Chi
Ψ ψ Psi	Ω ω Omega
obsolete letters
Ϝ ϝ Digamma	Ϻ ϻ San
Ϙ ϙ Qoppa	Ϡ ϡ Sampi

Mu (upper case Μ, lower case μ) is the 12th letter of the Greek alphabet. In the system of Greek numerals it has a value of 40. In the ISO 8859 character encodings standard, it has an ordinal decimal value of 230.

The lower-case letter μ is used as a special symbol in many academic fields.

• In mathematics:
  • the Möbius function in number theory
  • the population mean or expected value in probability and statistics
  • a measure in measure theory
• In measurement:
  • the SI prefix micro-, which represents one millionth, or 10⁻⁶
  • the micron, an old unit which corresponds to the micrometre (which is now denoted "µm")
• In classical physics and engineering:
  • the coefficient of friction
  • reduced mass in the two-body problem
  • permeability in electromagnetism
• In particle physics:
  • the elementary particle called the muon

Rarely, the letter μ may be used to distinguish one item represented by "m" from another (but a better option might be to use "m", "m", "m", etc.).

Letters that arose from the Greek Μ include the Latin M and Cyrillic М.

See also: Mu (letter), Academic field, Alpha (letter), Beta (letter), Character encodings, Chi (Greek letter), Classical physics, Coefficient of friction