1. the branch of governmental service that conducts secret investigations, esp. investigations into the military strength of other nations.
2. (caps.) the branch of the U.S. Department of the Treasury charged chiefly with the discovery and apprehension of counterfeiters and with providing protection for the president and the president's immediate family.
3. secret work for a government, esp. espionage.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.