Source : Webster's Revised Unabridged Dictionary (1913)

Harmonium \Har*mo"ni*um\, 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.

Source : WordNet®

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