The compartment of a starship where photon torpedoes are stored, prepared and loaded into their launch tubes, seen most memorably in the films featuring Kirk's Enterprise.
A torpedo bay is the long compartment that feeds a starship's torpedo launchers. Crew members there take casings from storage, arm them and send them along a track into the tube.
The refitted Enterprise's bay is shown in detail in Star Trek II: The Wrath of Khan, where trainees haul up the deck gratings to load by hand during the battle with the Reliant. At the end of the film the bay is the setting for Spock's funeral, and his body is fired from it in a torpedo casing towards the Genesis planet.
In Star Trek VI: The Undiscovered Country, Spock and McCoy work in the bay to fit a torpedo with sensing gear so that it will home on the exhaust of Chang's cloaked Bird-of-Prey. In Star Trek Into Darkness the bay of the alternate Enterprise holds the experimental torpedoes in which Khan's followers lie frozen.