A mess hall was a location on Federation and Klingon starships or starbases where crewmembers could relax and enjoy a meal or a quick snack in a social setting. An old Earth nautical term, for the mass dining room for ships crew.
It was also the place where officials, such as ambassadors of other worlds, were entertained. Gatherings were also held there.
Aboard J-class freighters, the mess hall was in room A3. The J-class mess hall had at least two circular windows and four tables. On the opposite wall from the windows was a counter, featuring a metallic, manual drinks dispenser. There was a pair of double-doors on each of the other sides of the room. Food available included strawberry shortcake nutri-paks. Drinks could be consumed in metallic mugs found at the counter. (ENT: "Horizon")
The Freedom-class had a combination of a mess/recreational hall, which was located on the forward end of the saucer section. It featured one table with at least six chairs and a leather couch across the room. (Star Trek Beyond)
Earth Starfleet NX-class starships of the 22nd century were outfitted with large mess halls. Boasting several large windows, the NX-class mess hall was a buffet-style environment; food prepared in the ship's galley was brought to the mess hall and stored in a serving case located on a bulkhead adjacent to a drink dispenser. (ENT: "Fortunate Son", "Acquisition", "Regeneration", et al.)
Aboard the starship Enterprise NX-01, the mess hall was converted into a theater on some Tuesday nights for "movie night". A large screen was erected in the mess hall, controllable from bridge stations on A Deck. (ENT: "Dear Doctor", et al.)
The mess hall of a Crossfield-class starship featured a food synthesizer and several small windows along the hull. The room also featured multiple tables and seats, which were colored white. Aboard the Crossfield-class USS Discovery in 2256, the mess hall's innermost wall featured drawings that comprised a memorial of starships lost in battle. (DIS: "Context Is for Kings")
Like much of the internal spaces aboard Defiant-class starships, the mess hall was compact, functional and had no windows. It was equipped with one food replicator. The mess hall could also be used for meetings with the ship's crew. (DS9: "Trials and Tribble-ations", "Valiant")
Several weeks later, the mess hall was the site of a meeting between Captain Sisko, Quark, and Hanok. Later, a Jem'Hadar torpedo impacted into the mess hall, but did not explode immediately. Quark and Hanok were able to defuse it. (DS9: "Starship Down")
On Intrepid-class vessels, the mess hall was on Deck 2, Section 13, with turbolift access in the corridor outside the starboard entrance. Some ships were equipped with four food replicators, with a private captain's dining room just behind the mess hall. The dining room could be reconfigured, and two of the replicators removed, to serve as a kitchen, as was done by Neelix aboard the USS Voyager. (VOY: "Caretaker", "Distant Origin", "Phage", "The Cloud") The USS Bellerophon's mess hall was also fitted in this manner. (DS9: "Inter Arma Enim Silent Leges")
Since replicator rations were in short supply aboard Voyager, Neelix supplemented food stocks with vegetables from the airponics bay along with food obtained from trading and scouting missions.
The mess hall aboard Voyager was host to many social gatherings, including diplomatic affairs, (VOY: "Someone to Watch Over Me") birthday parties, (VOY: "Latent Image") ping pong games, (VOY: "Relativity") and holiday celebrations. (VOY: "Homestead")
In mid-2378, after Neelix had departed Voyager, Chell wanted to take over duties in the mess hall. The ship returned to the Alpha Quadrant shortly following, and it is not known if Chell was given the position. (VOY: "Endgame")
Several Klingon Birds-of-Prey – such as the IKS Rotarran, the IKS Ch'Tang, and the IKS Pagh – were known to include mess halls. (TNG: "A Matter Of Honor"; DS9: "Soldiers of the Empire", "Sons and Daughters", "Once More Unto the Breach")