Meaning of M in English
( n. )
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.
Webster's Revised Unabridged English Dictionary. Пересмотренный полный словарь английского языка Webster. 2012