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

Moby Thesaurus II by Grady Ward, 1.0:

165 Moby Thesaurus words for "plenum":
   Copernican universe, Einsteinian universe, Indian file,
   Newtonian universe, Ptolemaic universe, air, all, all being,
   all creation, allness, array, articulation, assemblee, assembly,
   assignation, at home, atom, atomic particles, ball, bank, brawl,
   brute matter, building block, buzz, catena, catenation, caucus,
   chain, chain reaction, chaining, chemical element, colloquium,
   commission, committee, component, concatenation, conclave,
   concourse, congregation, congress, connection, consecution,
   constituent, continuum, conventicle, convention, convocation,
   cosmos, council, course, created nature, created universe,
   creation, cycle, dance, date, descent, diet, drone, earth,
   eisteddfod, element, elementary particle, elementary unit,
   endless belt, endless round, everything that is,
   expanding universe, festivity, fete, file, filiation, fire,
   forgathering, forum, fundamental particle, gamut, gathering,
   get-together, gradation, housewarming, hum, hyle, hypostasis,
   levee, line, lineage, macrocosm, macrocosmos, material,
   material world, materiality, matter, meet, meeting, megacosm,
   metagalaxy, molecule, monad, monotone, natural world, nature,
   nexus, omneity, panel, party, pendulum, periodicity,
   physical world, powder train, progression, prom,
   pulsating universe, queue, quorum, rally, range, rank, reception,
   recurrence, rendezvous, reticulation, rotation, round, routine,
   row, run, scale, seance, sequence, series, session, shindig,
   sidereal universe, single file, sit-in, sitting, soiree, spectrum,
   steady-state universe, string, stuff, substance, substratum,
   succession, sum of things, swath, symposium, synod, system,
   the four elements, thread, tier, totality, totality of being,
   train, turnout, unit of being, universe, water, whole wide world,
   wide world, windrow, world, world without end