- magic number (1937)
- make-or-break season (1937)
- manufacture (runs) (1898)
- mathematically eliminated (1916)
- meet the ball (1874)
- Mendoza Line(1982)
- Minor League (1884)
- minor leaguer (1885)
- mistake hitter (1959)
- Most Valuable Player (1870)
- mound (1903)
- moundsman (1906)
