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. (DS9: "Treachery, Faith and the Great River")
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. (DS9: "Whispers")
In 2373, a member of Starfleet became an informant for the Orion Syndicate when he met Raimus on Risa. (DS9: "Honor Among Thieves")
In 2374, Glinn Lasaran already worked for Starfleet as an Informant for several months. (DS9: "Change of Heart")
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. (DS9: "Inter Arma Enim Silent Leges")