Mole (espionage)

In espionage jargon a mole ("deep cover agent", or "sleeper") is a long-term spy. 
The spy is recruited "before" having access to secret intelligence. Later, if he gets into the target organization, he or she becomes active. 
However, it is popularly used to mean any long-term clandestine spy or informer in an organization, government or private. In police work, a mole is an undercover law-enforcement agent who joins an organization in order to collect incriminating evidence about its operations, and so bring its members to justice.
The term was introduced to the public by British spy novelist John Le Carré in his 1974 novel "Tinker, Tailor, Soldier, Spy".