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:

'Em \'Em\
   An obsolete or colloquial contraction of the old form hem,
   them. --Addison.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Em \Em\, n. (Print.)
   The portion of a line formerly occupied by the letter m, then
   a square type, used as a unit by which to measure the amount
   of printed matter on a page; the square of the body of a
   type.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Em- \Em-\
   A prefix. See En-.
   [1913 Webster]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

EM
       Extensions Manager (Apple)