secret service

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Intelligence-gathering activities conducted secretly by a government agency.
• noun A government agency engaged in intelligence-gathering activities.
• noun A branch of the US Treasury Department concerned especially with protection of the president.

from the GNU version of the Collaborative International Dictionary of English.

• 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A governmental agency that deals with espionage and other acts of secrecy.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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