A Gangster or mobster is a criminal who is a member of a crime organization , such as a gang . The terms are widely used in reference to members of gangs associated with American prohibition and the American offshoot of the Mafia , such as the Chicago Outfit or the Five Families . The terms `` gangster '' and `` mobster '' are mostly used in the United States to refer to members of criminal organizations associated with Prohibition or with an American offshoot of the Italian Mafia ( such as the Chicago Outfit , the Philadelphia Mafia , or the Five Families ) .