Source : Webster's Revised Unabridged Dictionary (1913)

Plenum \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.

Plenum \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.

Source : WordNet®

plenum
     n 1: a meeting of a legislative body at which all members are
          present; "the plenum will vote on all tax increases"
     2: an enclosed space in which the air pressure is higher than
        outside
     [also: {plena} (pl)]