NettetM - definition of M by The Free Dictionary. Also found in: Thesaurus, Medical, Legal, Financial, Acronyms, Encyclopedia, Wikipedia . M 1. also m. The symbol for the Roman numeral 1,000. M 2. abbr. 1. Printing em. 2. Bible Maccabees. 3. Mach number. 4. male. 5. married. 6. medium. 7. meridian. 8. Latin meridies (noon) 9. metal. 10. middle.