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.