Plenum

A compartment or chamber to which one or more air ducts are connected and that forms part of the air distribution system.

An air compartment or chamber including uninhabited crawl spaces areas above a ceiling or below a floor, including air spaces below raised floors of computer / data processing centers or attic spaces, to that one or more ducts are connected and that forms part of either the supply-air, return-air, or exhaust-air system, other than the occupied space being conditioned.