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.

Source: Webster's Revised Unabridged Dictionary (1913)
 

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

Source: Webster's Revised Unabridged Dictionary (1913)
 

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.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Em- \Em-\ A prefix. See En-.

Source: Webster's Revised Unabridged Dictionary (1913)