mag (1)EnglishOxford Dictionary of Englishmag1 /maɡ / ▸ noun informal 1 a magazine (periodical): the bestselling rock mag in Britain. 2 a magazine (of ammunition): I've only got one spare mag for the Browning. 3 [mass noun] magnesium or magnesium alloy. 4 a magneto. 5 magnitude (of stars or other celestial objects).