WordNet (r) 3.0 (2006):

mobster
    n 1: a criminal who is a member of gang [syn: gangster,
         mobster]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "mobster":
   Judas, bad person, bandit, betrayer, brigand, convict, criminal,
   crook, dacoit, deceiver, delinquent, desperado, desperate criminal,
   double-dealer, evildoer, felon, fugitive, gallows bird, gangster,
   gaolbird, hoodlum, jailbird, lawbreaker, malefactor, malevolent,
   malfeasant, malfeasor, misfeasor, mobsman, outlaw, public enemy,
   quisling, racketeer, scofflaw, sinner, swindler, thief, thug,
   traitor, transgressor, two-timer, villain, worker of ill,
   wrongdoer