Definition of plenum

i. A system of ventilation in which air is forced into an inclosed space, such as a room or a caisson, so that the outward pressure of air in the space is slightly greater than the inward pressure from the outside, and thus leakage is outward instead of inward.

ii. A mode of ventilating a mine or a heading by forcing fresh air into it.

iii. Use of compressed air to hold soil from slumping into an excavation.

Ref: Nichols, 1