The Collaborative International Dictionary of English v.0.48:

fifth column \fifth` col"umn\, n. [from a statement during the
   Spanish Civil War (1936) that the Falange had four columns of
   soldiers marching on the city, and a fifth column "already
   there" (i.e. sympathizers inside the Republican lines).]
   1. a group of persons inside the battle lines of a territory
      engaged in a conflict, who secretly sympathize with the
      enemy, and who engage in espionage or sabotage; --
      sometimes also referred to as a trojan horse.
      [RP]

   2. Hence, any faction of persons within a group who secretly
      sympathize with an enemy, especially those who engage in
      activities harmful to the group; an enemy in one's midst;
      a group of traitors.
      [RP]

WordNet (r) 3.0 (2006):

fifth column
    n 1: a subversive group that supports the enemy and engages in
         espionage or sabotage; an enemy in your midst [syn: fifth
         column, Trojan horse]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "fifth column":
   Judas-like, Trojan horse, Trojan-horse, betraying,
   collaborationist, collaborator, crypto, double-crossing,
   fellow traveler, fifth columnist, fraternizer, quisling,
   quislingistic, saboteur, security risk, subversive, traitorous,
   treasonable, treasonous, turncoat, two-timing, underground