The officer's lounge was a recreational area aboard Federation starships and installations. It was also used for official functions such as receptions and dinners (TOS: "Space Seed", "Journey to Babel", "Is There in Truth No Beauty?")
Whether or not enlisted crewmembers were allowed in was never established.
During the V'ger crisis in the 2270s, James T. Kirk had a talk with Spock and Dr. Leonard McCoy in the officer's lounge shortly after the USS Enterprise's refit. (Star Trek: The Motion Picture)
In 2285, Kirk and Fleet Admiral Harry Morrow had a meeting in the officer's lounge at Starfleet Headquarters. The lounge was visited by several Starfleet officers and their companions, such as an officer's wife. (Star Trek III: The Search for Spock)
The costume and costume tag worn by the officer's wife, played by extra Claudia Lowndes, revealed the location as the officer's lounge.