plenum...

According to the National Electric Code (NEC) a plenum is a "compartment or chamber to which one or more air ducts are connected and [which] forms part of the air distribution system." To qualify as a plenum, the space above an acoustic tile ceiling would have to extend above other rooms in the same building or be open to ducts connecting it to other parts of the building. The concern is that during a fire, if there is burning material in a plenum air space, smoke and fumes can travel through air ducts to the whole building. For this reason, there are codes to restrict the types of materials (such as wiring) that can be placed in the plenum.