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:

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):

organized crime
    n 1: underworld organizations [syn: organized crime,
         gangland, gangdom]

Moby Thesaurus II by Grady Ward, 1.0:

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