plenum

Short definition from Wiktionary

Noun

1. 1
   A space that is completely filled with matter.
2. 2
   A state of fullness, a great quantity (of something).
3. 3
   A legislative meeting (especially of the Communist Party) in which all members are present.
4. 4
   An enclosed space having greater than atmospheric pressure.
5. 5
   The space above a false ceiling used for cables, ducts etc.
6. 6
   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.