The Collaborative International Dictionary of English v.0.48:

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

   2. (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 Suppl.]

WordNet (r) 3.0 (2006):

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