em

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]