Though this word has several meanings, in the architecture and design world, a plenum is an air-filled space in a structure. More specifically, it’s a space that receives air from a heating / air-conditioning / ventilation units for distribution on the floor of a building. Plenums are made accessible through suspended ceiling systems (i.e. grid and tile), and they also house utilities like electricity and communications, as well as specialties like acoustical sound controls and security. Bonus info: Depending on local code, these items – as well as the plenum spaces themselves – must have certain fire ratings to prevent the spread of fire and smoke in a commercial building. Some light fixtures are “return” fixtures, meaning that they allow the flow of “return air” back into the plenum, rather than having to install return grilles.
See also: power poles