A mess hall was a facility on starships and facilities where a person can relax and enjoy a meal in a social setting.
On Intrepid-class starships, the mess hall is located on Deck 2, Section 18 and were equipped with four food replicators along the back wall where crewmembers could obtain their meals. In the case of the USS Voyager which became stranded in the Delta Quadrant, the replicators were removed and a space knocked through into the captain's personal dining room and a kitchen area was created where Neelix could cook "real" food to save the ship's power reserves. (VOY episodes: "Caretaker", "Phage" & "The Cloud")
As well as serving as a place where crewmembers could enjoy a meal they would offer serve as a place for diplomatic gatherings and social gatherings; including birthday parties, sporting tournaments and holiday celebrations. (DS9 episode: "Inter Arma Enim Silent Leges"; VOY episode: "Latent Image"; short story: "Isabo's Shirt")