An observation lounge is an area within a starship or starbase.
In the 23rd century, the observation lounge within a Constitution-class starship was a recreational area, offering views of nearby stellar phenomena as the ship passed through space. (EV comic: "Our Dearest Blood")
In the 24th century the observation lounge was the designation for the briefing room on Galaxy class and Sovereign class starships.
Deep Space 9's Promenade had an observation lounge. (DS9 novel: A Stitch in Time)
Observation lounge article at Memory Alpha, the wiki for canon Star Trek.