An informant was a person in a government or organization that provided information to an external source.
Coutu's Parada rebels had several informants in the Parada government by 2370. One of them relayed the rumor that Miles O'Brien had been abducted and replaced by a replicant, possibly to kill their delegation at a peace conference on Deep Space 9. (DS9: "Whispers")
In 2371, Enabran Tain mused during the Battle of the Omarion Nebula that there must have been an informant for the battle to be so devastating for the Romulan-Cardassian fleet. The informant was in fact Lovok. (DS9: "The Die is Cast")
When Benjamin Sisko approached Garak to steal secret data from Dominion Headquarters later that year, Sisko told him to use his contacts like a colleague from the Obsidian Order, an old friend or a reliable informant. (DS9: "In the Pale Moonlight")