‖n. [ L., fr. plenus full. ] 1. That state in which every part of space is supposed to be full of matter; -- opposed to vacuum. G. Francis. [ 1913 Webster ]
2. (Ventilation) A condition, as in an occupied room, in which the pressure of the air is greater than that of the outside atmosphere; as, a plenum may exist in a hall ventilated by a fan blower. [ Webster 1913 Suppl. ]