counterintelligence

counterintelligence
    n 1: intelligence activities concerned with identifying and
         counteracting the threat to security posed by hostile
         intelligence organizations or by individuals engaged in
         espionage or sabotage or subversion or terrorism
    

24 Moby Thesaurus words for "counterintelligence":
      bugging, cloak-and-dagger work, counterespionage,
      electronic surveillance, espial, espionage, following,
      intelligence, intelligence bureau, intelligence service,
      intelligence work, military intelligence, naval intelligence,
      observation, secret police, secret service, shadowing, spying,
      stakeout, surveillance, tailing, trailing, wiretap, wiretapping