The Collaborative International Dictionary of English v.0.48:

organized crime \organized crime\ n.
   Groups of persons organized for illegal purposes, such as
   bootlegging, conducting illegal gambling, loansharking,
   extortion, etc.; -- a general term encompassing most forms of
   criminal groups, but especially those that are consolidated
   into "families" more or less recognizing each other's
   different regions of operation; sometimes considered
   synonymous with the mafia or the syndicate.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Maffia \Maf"fi*a\, Mafia \Ma"fi*a\, n. [It. maffia.]
   1. A secret society which organized in Sicily as a political
      organization, but is now widespread among Italians, and is
      used to further or protect private interests, reputedly by
      illegal methods; called also the Sicilian Mafia.
      [WordNet sense 2]
      [Webster 1913 Suppl.]

   2. A group of loosely associated of criminal organizations in
      the United States, some having ties to the Sicilian Mafia,
      and organized in "families"; the term is applied to the
      entire group of organizations, or to any one local group.
      Also, loosely, organized groups of criminals anywhere, as
      the Russian mafia. [WordNet sense 1]

   Syn: syndicate, mob, Cosa Nostra, La Cosa Nostra, organized
        crime.
        [WordNet 1.6 + PJC]

   3. Any tightly knit group of trusted associates having strong
      control or influence in some area; as, Kennedy and his
      Irish Mafia. [informal] [WordNet sense 3]
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Mafia \Maf"i*a\, Mafioso \Ma`fi*o"so\, n.
   see maffia, maffioso.
   [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Syndicate \Syn"di*cate\, n. [Cf. F. syndicat, LL. syndicatus.]
   1. The office or jurisdiction of a syndic; a council, or body
      of syndics. --Bp. Burnet.
      [1913 Webster]

   2. An association of persons officially authorized to
      undertake some duty or to negotiate some business; also,
      an association of persons who combine to carry out, on
      their own account, a financial or industrial project; as,
      a syndicate of bankers formed to take up and dispose of an
      entire issue of government bonds.
      [1913 Webster]

   3. A more or less organized association of criminals
      controlling some aspects of criminal activity, in a
      specific area or country-wide; -- used loosely as a
      synonym for organized crime or the mafia.
      [PJC]

   4. (Journalism) a commercial organization that purchases
      various journalistic items, such as articles, columns, or
      comic strips, from their individual creators, and resells
      them to newspapers or other periodicals for simultaneous
      publication over a wide area.
      [PJC]

WordNet (r) 3.0 (2006):

Mafia
    n 1: a crime syndicate in the United States; organized in
         families; believed to have important relations to the
         Sicilian Mafia [syn: Mafia, Maffia, Cosa Nostra]
    2: a secret terrorist group in Sicily; originally opposed
       tyranny but evolved into a criminal organization in the
       middle of the 19th century [syn: Mafia, Maffia, Sicilian
       Mafia]
    3: any tightly knit group of trusted associates [syn: mafia,
       maffia]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "Mafia":
   Black Hand, Cosa Nostra, black market, bootlegging, gambling,
   gangdom, gangland, gray market, illegal commerce,
   illegal operations, illegitimate business, illicit business,
   loan-sharking, moonshining, narcotics traffic, organized crime,
   prostitution, protection racket, racket, shady dealings, the Mafia,
   the mob, the rackets, the syndicate, the underworld,
   traffic in women, usury, white slavery