(1) Plenum is the air-handling space such as that found above drop-ceiling tiles or in raised floors. Also, plenum is a fire-code rating for indoor cable. (2) The air return path of a central air handling system, either ductwork or open space over a dropped ceiling.

Plenum Cable is coaxial or Fiber cable that is coated with a fire-retardant Coating SO that in case of a fire it does not give off toxic gasses and smoke as it burns.