The harmonium or pump organ is a musical instrument in the percussion family. It is a type of reed organ with a tube and a frame, which produces sound when air is pumped through it. The harmonium can be used as a standalone instrument, or it can be part of a larger system, such as a church. It is most commonly played with a keyboard, although it can be played with several different instruments, including the xylophone, glockenspiel, drum kit, and marimba. The sound it produces is unique and characteristic, and it can be used to create a wide variety of musical effects.

The pump organ has been used in many different genres of music, from classical to jazz and blues.