A bulletin was a report that gave news of significant and immediate interest.
In 2267, Starfleet Command sent out a bulletin stating that the Romulans had broken Code 2. When Nyota Uhura reminded a prematurely aging Captain James T. Kirk of this fact, he changed his order to Code 3. Later, during Kirk's competency hearing, Spock pointed out that Kirk had signed his initials to the bottom of the document – indicating he had read it prior to giving the order – as proof that Kirk was becoming unfit to command. (TOS: "The Deadly Years")
In 2371, Deep Space 9 received a bulletin on a Ferengi who was apparently a tongo hustler. A week later, during Jadzia Dax's zhian'tara, Odo, manifesting Curzon Dax's personality, mentioned the bulletin and thought it would be great fun to clean him out. (DS9: "Facets")