infiltration

The covert insertion of an officer, source or other operative into a target area.

The CIA glossary defines infiltration as the covert movement of an officer, source or other operative into a target area in order to obtain information or carry out an operation. The reverse movement is exfiltration.¹

An example from the Second World War: according to the CIA, more than 90 Jedburgh teams (of two to four men) were parachuted into France in 1944, the first on the night before D-Day. Their task was to coordinate supply drops, direct the Resistance and carry out sabotage.²

In intelligence parlance the word also has a second meaning: penetrating an organisation. The International Spy Museum describes a mole as an agent who penetrates a foreign service by taking a job in it.³