A plenum is the (technically impossible to create) antithesis of a vacuum, in other words completely filled space.

Plenums are 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.

Plenum spaces in buildings are spaces for air circulation in air conditioning systems. Structured cabling and fibre optic cable is frequently laid in plenum spaces. To prevent excessive fire risk, there are strict restrictions on the kind of cable installations which are permitted in plenum spaces. See Plenum cable.

A plenum refers to 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.