fifth column

from 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}]

from 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]

from 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