| n. | 1. | That state in which every part of space is supposed to be full of matter; - opposed to |
| 2. | (Ventilation) A condition, as in an occupied room, in which the pressure of the air is greater than that of the outside atmosphere; |
| Noun | 1. | plenum - a meeting of a legislative body at which all members are present; "the plenum will vote on all tax increases" |
| 2. | plenum - an enclosed space in which the air pressure is higher than outside |