The USS Enterprise had a wardroom, which was used to entertain visiting dignitaries. Charlie Evans walked past a wall sign that read "Ward Room B4" just before he transformed a young woman into an old lady crewwoman because he thought she was laughing at him. (TOS: "Charlie X") The wardroom was also used, with stewards in attendance, when the Enterprise's senior officers entertained Khan Noonien Singh to welcome him to the 23rd century. (TOS: "Space Seed")
In a similar fashion, the USS Enterprise-A also had a wardroom, called the mess hall or officer's mess. It was used to entertain Chancellor Gorkon and his party in 2293, after the Klingon moon, Praxis, had exploded. (Star Trek VI: The Undiscovered Country)
In addition, Romulan D'deridex-class warbirds had a wardroom which was located to the starboard rear of the bridge. It was used as a private dining room for the ship's commander and senior staff. (TNG: "Face of the Enemy")
Deep Space 9 also had a wardroom which was used in a similar fashion to a briefing room on Starfleet vessels, though its large area allowed it to be used for a variety of other purposes. While used by Benjamin Sisko to brief his officers, it was also used during events such as the Bajoran Gratitude Festival, Jadzia Dax's zhian'tara, Sisko's promotion to captain, (DS9: "Fascination", "Facets", "The Adversary")
Rom and Leeta's wedding ceremony and reception, Sisko's posting of casualty lists during the Dominion War, and the signing ceremony for the treaty that ended the war. (DS9: "Call to Arms", "In the Pale Moonlight", "The Siege of AR-558", "What You Leave Behind")
Unlike many other parts of the station, which maintained their Cardassian era displays, the wardroom contained an LCARS display monitor. During the Dominion occupation of the station the wardroom's display was replaced by a Cardassian-style display. (DS9: "A Time to Stand")
The TOS wardroom was a redress of the briefing room, while the Undiscovered Country wardroom was a re-dress of the USS Enterprise-D observation lounge. The DS9 wardroom made its first appearance in the DS9 Season 3 premiere, "The Search, Part I". However, its appearance in "Wrongs Darker Than Death or Night" indicates that the wardroom existed on Terok Nor during the Bajoran Occupation. In the British Royal Navy, the wardroom is the mess cabin of naval commissioned officers above the rank of sub-lieutenant. It is a place where these officers can relax when off-duty though there is a strict etiquette to be followed. The captain of the ship is not usually a member of the wardroom, though can be invited in by its members.