Plenum

Plenum may refer to:

the antithesis of a vacuum; in other words, completely filled space.
the pressures slightly above atmospheric, usually inside air or gas systems as the result of the action of fans or blowers. The plenum is measured in small units of pressure such as pascals (Pa) or ounces-force per square inch or millimeters of mercury.
a legislative meeting in which all members of the body are present and voting. Frequently this is used to refer to a full meeting of a Communist Party central committee. Compare to quorum.
a second set of bellows in devices such as hand or foot pumped organs with a weight on the top which acts as an air reservoir.

Plenum spaces in buildings are spaces for air circulation in air conditioning systems.
Plenum cable refers to structured cabling permitted under building codes for plenum spaces.
where the space above a suspended ceiling is continuous and part of the supply or return air systems of a buildings HVAC system.

Also a term used in Heating and Ventilation, regarding the manufacture of Plenum Boxes, essentially a grill box with a splitter inside.