1. (noun)m a quadrat, the face or top of which is a perfect square; also, the size of such a square in any givensize 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
2. (noun)m a brand or stigma, having the shape of an M, formerly impressed on one convicted of manslaughter and admitted to the benefit of clergy
3. m m, the thirteenth letter of the English alphabet, is a vocal consonant, and from the manner of its formation, is called the labio-nasal consonant. See Guide to Pronunciation, // 178-180, 242
4. m as a numeral, M stands for one thousand, both in English and Latin
Definitions of m 
