The space in which air conditioning flows. Can be either through ducts in the ceiling or floor, or the entire underside of your raised floor. It is especially fun to remove a tile and stick your head under the floor; very refreshing, especially after a really hot muggy Miami day. Cable is sometimes plenum rated, which means that fire codes allow it to be installed within a plenum. Plenum rating is important because a fire in an area where air is moving will spread very rapidly.