Plenum

Plenum is a term with Latin roots meaning "full" or "filled." In modern usage it describes a space or chamber that is fully occupied by air, gas, or matter, or a meeting of the entire membership of a deliberative body. The sense depends on context, but all uses share the idea of a complete, centralized space for distribution, collection, or council.

In building services and mechanical systems, a plenum is an air-handling or air-distribution chamber. It can be a dedicated room, a ceiling or floor cavity, or a duct-connected box that stores conditioned air before it enters distribution ducts. Plenums help balance pressure and uniform flow and may also serve as a return-air space in a ventilation system.

In parliamentary and organizational contexts, a plenary or plenum is the full assembly of members. Plenary sessions are when all members meet to discuss and vote on business, as opposed to committee meetings or sub-assemblies. The term is used in various legislatures, international bodies, and conferences.

In philosophy and early science, plenum referred to a space fully filled with matter, as opposed to a vacuum. The concept has influenced debates about the nature of space, matter, and the fate of vacuums, though modern physics typically treats vacuums as voids within quantum fields.

Additionally, "plenum" appears in technical uses such as a plenum chamber in internal combustion engines and gas-turbine combustors, and in architecture and fire-safety codes describing the air-space above ceilings or below floors used for distribution.