The Collaborative International Dictionary of English v.0.48:

Secret service \Se"cret serv"ice\
   The detective service of a government. In the United States,
   in time of peace the bureau of secret service is under the
   treasury department, and in time of war it aids the war
   department in securing information concerning the movements
   of the enemy.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Secret Service
    n 1: the United States intelligence agency that protects current
         and former presidents and vice presidents and their
         immediate families and protects distinguished foreign
         visitors; detects and apprehends counterfeiters; suppresses
         forgery of government securities and documents [syn:
         United States Secret Service, US Secret Service,
         USSS, Secret Service, SS]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "Secret Service":
   Bow Street runner, FBI, FBI agent, Federal, G-man, Sherlock Holmes,
   T-man, detective, fed, hotel detective, house detective,
   house dick, inquiry agent, investigator, narc, operative,
   plainclothesman, police detective, private detective,
   private investigator, revenuer, sleuth, store detective,
   treasury agent



Moby Thesaurus II by Grady Ward, 1.0:

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