1. 1. Intelligence-gathering activities conducted secretly by a government agency.
   2. A government agency engaged in intelligence-gathering activities.
2. Secret Service A branch of the U.S. Treasury Department concerned especially with protection of the President.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.