plenum: Meaning and Definition of

ple•num

1. the state or a space in which a gas, usually air, is contained at a pressure greater than atmospheric pressure.
2. a full assembly, as a joint legislative assembly.
3. a space, usually above a ceiling or below a floor, that can serve as a receiving chamber for air that has been heated or cooled to be distributed to inhabited areas.
4. the whole of space regarded as being filled with matter (opposed to vacuum).