The Collaborative International Dictionary of English v.0.48:

Harmonium \Har*mo"ni*um\ (h[aum]r*m[=o]"n[i^]*[u^]m), n. [NL.
   See Harmony. ]
   A musical instrument, resembling a small organ and especially
   designed for church music, in which the tones are produced by
   forcing air by means of a bellows so as to cause the
   vibration of free metallic reeds. It is now made with one or
   two keyboards, and has pedals and stops.
   [1913 Webster]

WordNet (r) 3.0 (2006):

harmonium
    n 1: a free-reed instrument in which air is forced through the
         reeds by bellows [syn: harmonium, organ, reed organ]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "harmonium":
   Hammond organ, baroque organ, calliope, calliophone, choralcelo,
   electric organ, electro-pneumatic organ, hydraulic organ, melodeon,
   melodica, orchestrelle, organ, organophone, pipe organ, reed organ,
   seraphine, steam organ, symphonion, tracker-action organ,
   tubular-pneumatic organ, vocalion