The Collaborative International Dictionary of English v.0.48:

M \M\, n.
   1. (Print.) A quadrat, the face or top of which is a perfect
      square; also, the size of such a square in any given size
      of type, used as the unit of measurement for that type:
      500 m's of pica would be a piece of matter whose length
      and breadth in pica m's multiplied together produce that
      number. [Written also em.]
      [1913 Webster]

   2. (law) A brand or stigma, having the shape of an M,
      formerly impressed on one convicted of manslaughter and
      admitted to the benefit of clergy.
      [1913 Webster]

   M roof (Arch.), a kind of roof formed by the junction of
      two common roofs with a valley between them, so that the
      section resembles the letter M.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

M \M\ ([e^]m).
   1. M, the thirteenth letter of the English alphabet, is a
      vocal consonant, and from the manner of its formation, is
      called the labio-nasal consonant. See Guide to
      Pronunciation, [sect][sect] 178-180, 242.
      [1913 Webster]

   Note: The letter M came into English from the Greek, through
         the Latin, the form of the Greek letter being further
         derived from the Ph[oe]nician, and ultimately, it is
         believed, from the Egyptian. Etymologically M is
         related to n, in lime, linden; emmet, ant; also to b.
         [1913 Webster] M is readily followed by b and p. the
         position of the lips in the formation of both letters
         being the same. The relation of b and m is the same as
         that of d and t to n. and that of g and k to ng.
         [1913 Webster]

   2. As a numeral, M stands for one thousand, both in English
      and Latin.
      [1913 Webster]

The Free On-line Dictionary of Computing (17 December 2009):

M

   1. Alternative name for MUMPS.

   2. A C-like language from Silicon Compiler Systems for
   multilevel hardware description.  It is currently available
   in the GDT package from Mentor Graphics.

   [Jargon File]

   (1994-10-26)

   3.  The abbreviated for of mega-.

   (1995-01-10)


The Jargon File (version 4.4.7, 29 Dec 2003):

M
 pref.

    [SI] See quantifiers.