Backroom

n. room situated in the rear section of a building or house

For the photography term, please see: Darkroom A darkroom or dark room is a darkened room, sometimes located in a nightclub, gay bathhouse or sex club, where sexual activity can take place. When located in bars, dark rooms are also known as backrooms or blackrooms. The lighting in dark rooms ranges from pitch black to dimly lit. Some dark rooms incorporate black, blue, or red lights.
See more at Wikipedia.org...

Noun
1. the meeting place of a group of leaders who make their decisions via private negotiations
(hypernym) facility, installation

the room from which client personnel observe and listen to focus group proceedings through a one-way mirror. Also

any area where activity takes place other than the at the customer interface.  A temporary storage area for retail inventory, equipment and supplies.