Engineering circuit bay
The engineering circuit bay was a designation associated with certain Jefferies tubes in  starships.

In 2267, the Tellarite ambassador Gav was found dead in the engineering circuit bay located on Deck 11, Section A3. ()

Background information 
"Engineering circuit bay" appears on the wall plaque beside the "Jefferies tube" in several (but not all) scenes wherein characters had to enter the tubes. This suggests there could be more than one of these circuit bays in Constitution ships and/or not all Jefferies tubes necessarily led to engineering circuit bays.

According to Matt Jefferies' original schematics, the (only) engineering circuit bay was located at the bottom of the main dorsal junction.

Spacecraft sections