A harmonium is a free-standing keyboard instrument similar to a reed organ. Sound is produced by air being blown through sets of free reeds, resulting in a sound similar to that of an accordion. The air is usually supplied by bellows operated by the foot, hand, or knees.
In North America, the most common pedal-pumped free-reed keyboard instrument is known as the "American reed organ", (or "parlor organ", "pump organ", "cabinet organ", "cottage organ", etc.) and along with the earlier melodeon, is operated by a suction bellows where air is sucked through the reeds to produce the sound. A reed organ with a pressure bellows that pushes the air through the reeds is referred to as a "harmonium".
In much of Europe, the term harmonium is used to describe all pedal-pumped keyboard free-reed instruments, making no distinction whether it has a pressure or suction bellows.
In India, the term generally refers to a hand-pumped instrument.
Read more about Harmonium: History, Construction, India and Pakistan, Harmonium Acoustics, Classical Repertoire (Western Music), Nordic Folk Music, Use in Western Popular Music