The Collaborative International Dictionary of English v.0.48:

insider \insider\ n.
   an officer of a corporation or others who have access to
   private information about the corporation's operations,
   especially information relating to profitability.

   Note: An insider is forbidden by U. S. securities laws to
         trade stock in publicly owned corporations based on the
         private information. The definition of insider for the
         purpose of securities law has changed in the late 29th
         century to become more inclusive, whereas it initially
         was applied only to officers of a corporation.
         [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

insider
    n 1: an officer of a corporation or others who have access to
         private information about the corporation's operations

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "insider":
   Greek, affiliate, associate, belonger, brother, card-carrier,
   card-carrying member, cardholder, charter member, clubber, clubman,
   clubwoman, committeeman, comrade, confidential information,
   conventioneer, conventioner, conventionist, dues-paying member,
   enlistee, enrollee, fellow, fraternity man, guildsman,
   honorary member, initiate, inside dope, inside information, joiner,
   life member, member, one of us, pipeline, pledge, sister, socius,
   sorority girl, sorority woman, the lowdown