资料来源 : Webster's Revised Unabridged Dictionary (1913)

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}.]

   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.

   {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.