/plee"neuhm, plen"euhm/ , n. , pl. plenums, plena /plee"neuh, plen"euh/ .
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 ).
[ 1670-80; plenus FULL, in the phrase plenum ( spatium ) FULL (space) ]