A messenger was someone who delivered a message on behalf of another.
A popular idiom, "Don't kill the messenger," (VOY: "Relativity", "Favorite Son") originated from Sophocles' Oedipus Rex. (VOY: "Human Error")
Messenger was one of the words Data used in 2370 while interpreting unknown symbols. (TNG: "Masks")
In the story of "Rumpelstiltskin", the queen's messenger witnessed the dwarf dancing with glee over the queen being unable to guess his name. (DS9: "If Wishes Were Horses")
The Grand Proxy was the Grand Nagus's official messenger. When Neelix pretended to be this individual in 2373 to thwart Kol and Arridor, the latter Ferengi created a new rule to handle the situation: "When the messenger comes to appropriate your profits, kill the messenger." (VOY: "False Profits")