The conference room directly behind the main bridge of the Enterprise-D and later ships, with a long table and wide windows, where the senior staff meet.
The observation lounge is the meeting room on Deck 1 of the Enterprise-D, reached from doors at the back of the bridge. Its long curved table seats the senior officers, a wall display is used for briefings, and a row of tall windows looks aft over the ship's hull.
It is one of the most used sets of The Next Generation. Picard gathers his staff there in almost every episode to weigh options, and it hosts negotiations and hearings as well, including much of the planning in The Best of Both Worlds.
The Enterprise-E has a similar room. In Star Trek: First Contact, Picard and Lily argue there over his refusal to abandon the ship to the Borg, and he smashes the case of gold model starships on its wall in his anger. The restored Enterprise-D lounge is seen again in the last season of Star Trek: Picard.