Mu (letter)

Greek letters
Α α Alpha;	Β β Beta;
Γ γ Gamma;	Δ δ Delta;
Ε ε Epsilon;	Ϝ ϝ Digamma;
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;	Ϻ ϻ San;
Ϙ ϙ Qoppa;	Ρ ρ Rho;
Σ σ Sigma;	Τ τ Tau;
Υ υ Upsilon;	Φ φ Phi;
Χ χ Chi;	Ψ ψ Psi;
Ω ω Omega;	Ϡ ϡ Sampi;
Greek alphabet

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.

The lower-case letter μ is used as the symbol for:

• The Möbius function in mathematics.
• The SI prefix micro-, which represents one millionth, or 10⁻⁶ (in fact, the "micro sign" (µ) is used for this, but it is very similar).
• Formerly, the micron, an old unit corresponding to the micrometer.
• The coefficient of friction, ie. the ratio of the limiting frictional force between two surfaces against the normal contact force.
• The elementary particle called muon in particle physics.
• Reduced mass in the two-body problem in physics.
• Rarely, to distinguish between one item represented by m and another (but m might be an easier option).
• The permeability in physics and electrical engineering.