Informant

An informant is a person in a government or organization that provides information to an external source.

Gul Russol was Odo's most reliable informant in the Cardassian government. 

Coutu's Parada rebels had several informants in the Parada government. One of them was able to tell them of a government plot to use a replicant of Miles O'Brien to kill their delegation at a peace conference on Deep Space 9. 

In 2373, a member of Starfleet became an informant for the Orion Syndicate when he met Raimus on Risa. 

In 2374, Glinn Lasaran already worked for Starfleet as an Informant for several months. 

Koval, the chairman of the Tal Shiar, passed information to Starfleet since the start of the Dominion War and may have worked even longer for Section 31.