an agent of a secret service.
(loosely) a person employed to collect the military secrets of one country and relay them to another, especially a person living in a foreign country while so working for his or her own nation; spy.
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Origin of secret agent
First recorded in 1830–40
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for secret agent
a person employed in espionage
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012