A double agent or mole was a spy recruited from within an agency by another agency to report on activities thereof.
In 2375, a member of Species 8472 impersonating Boothby asked Chakotay if he was a Romulan double agent, as he had just arrived from the Neutral zone and planned to enter Logistical Support, a secure area, with a holo-imaging device. (VOY: "In the Flesh")
Also that year, Koval acted as a Starfleet mole for Section 31. They planned to insert him into the Romulan Continuing Committee following the removal of Kimara Cretak from consideration for a seat thereon. (DS9: "Inter Arma Enim Silent Leges")