noun

• (physics) A space that is completely filled with matter.
• (figuratively) A state of fullness, a great quantity (of something).
• A legislative meeting (especially of the Communist Party) in which all members are present.
• An enclosed space having greater than atmospheric pressure.
• The space above a false ceiling used for cables, ducts etc.
• (computing) A type of network cabling which satisfies plenum-ratings issued by the National Electrical Code. These cables produce less smoke and fumes in the event of fire.