Ple"num (?), n. [L., fr. plenus full.]
That state in which every part of space is supposed to be full of matter; -- opposed to vacuum. G. Francis.
© Webster 1913
Ple"num, n. (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