Har*mo"ni*um (här*mō"n&ibreve;*ŭm), 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.