The thirteenth letter of the Latin alphabet, M, represents the bilabial nasal consonant sound [m] in Classical languages as well as the modern languages. It derives its shape from the Greek &Mu or &mu. Semitic Mem originally pictured water, in all probability.

Mike represents the letter M in the NATO phonetic alphabet.

In context, M is also:

• As M, mega, an SI prefix meaning 10⁶ = 1,000,000 (one million) or a binary prefix used in computing to mean 2²⁰ = 1,048,576.
• As M, million, or millions, when used after certain currency expressions, such as $25M ("twenty-five million dollars).
• As m, milli, an SI prefix meaning 1/1000.
• As m, an SI unit for length, see metre.
• The Roman numeral for 1000 (mille in Latin).
• An abbrevation for "Messier" in designations for the Messier Deepsky Objects
• The symbol for molarity (M) or molality (m) of solutions in chemistry.
• The name of a 1931 movie directed by Fritz Lang about a serial killer who preys on children. There is also a lesser-known 1951 movie.
• M, the codename of the head of MI6 in the James Bond books and films
• A member of "Generation X", a team of younger mutants affiliated with and coached by some of the former "X-Men".
• the name of the artist M, that performed the song "Pop Muzik".
• -M-, the artist name of the French singer Matthieu Chédid.

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