(covers information from several alternate timelines)
Propaganda was a message that was used, especially in times of war, to influence the opinions of a populace.
In 2155 in the mirror universe, when Lieutenant and captain's woman Hoshi Sato told Captain Maximilian Forrest that she had heard the battle at Tau Ceti had gone well, Forrest replied that she had been listening to propaganda and that the battle had actually gone very badly. (ENT: "In a Mirror, Darkly")
Propaganda produced by the Klingon Empire in the 23rd century claimed that the United Federation of Planets committed atrocities against its prisoners, placing them in death camps and torturing them for their scientific and military secrets. (TOS: "Day of the Dove")
The Cardassian Union has used propaganda to influence its citizens for a long time. Propaganda was often broadcast over large viewscreens. Dukat's first speech as the leader of the Cardassian Union was shown in such a way. (DS9: "Tribunal", "Second Skin", "By Inferno's Light")
In 2373, Kira Nerys hoped that Tekeny Ghemor would be able to "issue corrections" to the propaganda of the Cardassian Union. Unfortunately, Ghemor died of Yarim Fel Syndrome soon after he visited Kira on Deep Space 9. (DS9: "Ties of Blood and Water")
In early 2374, Jake Sisko was concerned that instituting Weyoun's proposed changes to his Federation News Service articles would make them "Dominion propaganda". Weyoun claimed that he "would never dream of asking" him to write such a thing. (DS9: "A Time to Stand")