In the fictional Star Trek universe, the Battle Bridge is the command section located in the secondary hull of starships possessing a separable saucer section. In scenarios where this becomes necessary, such as combat, the battle bridge is used to control the main body of the starship.
The battle bridge also acts as an auxiliary control center for the starship, so if the main bridge is disabled, the entire ship can be controlled from the battle bridge. The battle bridge is the successor to the earlier idea of an "auxiliary control room" on the original Enterprise.
On Galaxy class starships, the battle bridge is located at the top of the secondary hull, on deck eight. It can be accessed by regular turbolifts (although access was restricted to authorized personnel only). There is also an emergency turbolift that runs directly from the main bridge to the battle bridge, giving the command staff quick and direct access to the battle bridge. The battle bridge has a ready room for the captain, it is a smaller and less elaborate version of the ready room on the main bridge. Like the main bridge, the battle bridge is also a self contained, plug-in module. This way, the battle bridge can be swapped out.
The battle bridge of the Enterprise-D has been seen three times, including the pilot episode Encounter at Farpoint and the first season episode The Arsenal of Freedom, as well as in a heavily remodeled form during the fourth season opening episode - The Best of Both Worlds, Part II. The Next Generation technical manual says that between the first and fourth seasons the battle bridge module was swapped out during a maintenance layover.