The Collaborative International Dictionary of English v.0.48:

mag \mag\ n.
   Shortened form of magazine, the periodic paperback
   publication. [slang]
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

mag
    n 1: a periodic publication containing pictures and stories and
         articles of interest to those who purchase it or subscribe
         to it; "it takes several years before a magazine starts to
         break even or make money" [syn: magazine, mag]