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Look up Μ, μ in Wiktionary, the free dictionary.

Greek alphabet
Αα Alpha	Νν Nu
Ββ Beta	Ξξ Xi
Γγ Gamma	Οο Omicron
Δδ Delta	Ππ Pi
Εε Epsilon	Ρρ Rho
Ζζ Zeta	Σσ Sigma
Ηη Eta	Ττ Tau
Θθ Theta	Υυ Upsilon
Ιι Iota	Φφ Phi
Κκ Kappa	Χχ Chi
Λλ Lambda	Ψψ Psi
Μμ Mu	Ωω Omega
Obsolete letters
Ϝϝ Digamma	Ϙϙ Qoppa
Ϛϛ Stigma	Ϡϡ Sampi
Ϻϻ San	Ϸϸ Sho
Greek diacritics

For other uses, see Mu.

Mu (uppercase Μ, lowercase μ) is the 12th letter of the Greek alphabet. In the system of Greek numerals it has a value of 40. Mu was derived from the Egyptian hieroglyphic symbol for water () which had been simplified by the Phoenicians and named after their word for water, to become Mem . Letters that arose from Mu include the Roman M and the Cyrillic letter Em (М, м).

The word mu (pronounced /muː/ or /mjuː/ in English and /mi/ by modern Greeks) is written μῦ in traditional Greek polytonic orthography. In Modern Greek it is sometimes written μύ (mi).

The letter mu appears in conjunction with alpha and omega to signify the "beginning, middle (meson) and end", a phrase found in an Orphic verse describing Zeus, and later adopted to describe both Jehovah and Jesus.

In Aeschylus' Eumenides, the repeated moaning of the letter mu is the sound made by the sleeping Furies as the ghost of Clytemnestra begins to invoke them. It again appears as an ominous mantra in a 10th century Coptic papyrus, containing a Christian injunction against perjurers that invokes the angel Temeluchos:

I adjure you by the seven perfect letters, ΜΜΜΜΜΜΜ. You must appear to him, you must appear to him. I adjure you by the seven angels around the throne of the father.

The lower-case letter mu is used as a special symbol in many academic fields. The upper case Mu isn't generally used in this way because it is normally indistinguishable from the Latin M.

• In mathematics:
  • the Möbius function in number theory
  • the integrating factor in ordinary differential equations
  • 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
  • Poisson's ratio
  • reduced mass in the two-body problem
  • permeability in electromagnetism
  • dynamic viscosity in fluid mechanics
• In inorganic chemistry:
  • the prefix given in IUPAC nomenclature for a bridging ligand.
• In particle physics:
  • the elementary particle called the muon
• In Pharmacology:
  • an important opiate receptor
• In thermodynamics:
  • the chemical potential of a system or component of a system.

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

In Unicode, the upper and lower case mu are encoded at U+039C and U+03BC respectively. In ISO 8859-7 they are encoded at CC_HEX and EC_HEX. The micro sign or micron is considered a distinct character from the Greek alphabet letter by Unicode for historical reasons (although it is written the same way, that is, shares a glyph) and is found at U+00B5 as well as position B5_HEX in ISO 8859-1, 3, 8, 9, 13 and 15. ISO-8859-5 also has a character that looks somewhat like lower case mu at E6_HEX but this is actually supposed to be the Cyrillic small letter tse.

When Alt-0181 or the DOS legacy Alt+230 or Alt+(any multiple of 256 added to 230 or to 0181 and including the leading zero) is typed into an editable field using the number pad in Microsoft Windows, the µ symbol appears. In HTML code, the µ symbol is represented by "µ" and "µ".

Because µ is the abbreviation for the Metric System prefix -micro, the symbol is used in many word plays about the field of micro-computing. For example, the symbol is used in the name and logo of the popular bittorrent client, µTorrent.

• Mike Paradinas, a British electronic musician, uses the letter in his stage name μ-ziq (pronounced music).
• Typing the numbers (4,8,15,16,23,42) from the fictional TV series Lost resulted in the character "µ". In Microsoft Windows, holding down the "Alt" key while entering these numbers results in the character appearing wherever your cursor is.
• µ is the SI prefix for micro and therefore sometimes used as an abbreviation or word-play standing for "micro", such as in "µ-soft", or µTorrent, a torrent client.

• Moralia, by Plutarch
• Ancient Christian Magic, by M. Meyer and R. Smith (ed.), Princeton University Press, ISBN 0-691-00458-7