A person sent secretly into an enemy's camp, territory, or fortifications, to inspect his works, ascertain his strength, movements, or designs, and to communicate such intelligence to the proper officer.
secretly collect sensitive or classified information; intentionally do something in a hidden way that people will not know about, "spy for the Russians using a spy camera".
Spy magazine was a satirical monthly founded in 1986 by Kurt Andersen and E. Graydon Carter, who served as its first editors, and Thomas L. Phillips, Jr., its first publisher.
Spy is a British television programme originally made by Wall to Wall for BBC Three in 2004. It has been one of the most-exported UK television shows of the present decade; according to the Producers' Alliance for Cinema and Television (PACT), it had been sold to 129 countries by April 2005.
A class or method that mimics an object under test, but whose only function is to report on values observed. So a spy is a somewhat elaborated stub or degenerate mock object.