Usually refers to a type of reed organ that generates sound with bellows (such as the harmonium, for which it is often a synonym), although strictly speaking the smaller pipe organs known as the positive and portative also had bellows.

A harmonium-like instrument was first produced around 1810 and derives from the earlier regal. Later developments included the idea of using suction bellows, notably in the usual type of 19th century American reed organ (confusingly often called a melodeon).

The harmonium became largely superseded with the introduction of electronic organs.