A battle bridge was a starship section, a control area that served as a ship's bridge during a battle situation.
History and specifics
The battle bridge was kept as a separate area from the main bridge for situations where a starship would undergo saucer separation or for situations where a separate area was needed to be used as an auxiliary control bridge. (TNG episodes: "Encounter at Farpoint", "The Arsenal of Freedom", "Heart of Glory", "The Best of Both Worlds")
On Galaxy-class starships the battle bridge was also modular, much like the main bridge. During her operational lifetime at least two different battle bridge modules were installed on the USS Enterprise. (TNG reference: Star Trek: The Next Generation Technical Manual)