The organ (from Greek όργανον organon, "organ, instrument, tool"), is a keyboard instrument of one or more divisions, each played with its own keyboard operated either with the hands or with the feet. The organ is a relatively old musical instrument in the Western musical tradition, dating from the time of Ctesibius of Alexandria who is credited with the invention of the hydraulis. By around the 8th century it had overcome early associations with gladiatorial combat and gradually assumed a prominent place in the liturgy of the Catholic Church; subsequently it has re-emerged as a secular and recital instrument.

Pipe organs use wind moving through pipes to produce sounds. Since the 16th century, pipe organs have used various materials of pipes, which can vary widely in timbre and volume. The pipes are divided into ranks and controlled by the use of hand stops and/or combination pistons. Although the keyboard is not expressive as on a piano and does not affect dynamics (pressing a key only turns the sound on or off), some divisions may be enclosed in a swell box, allowing the dynamics to be controlled by shutters. Some special registers with free reed pipes are expressive. These instruments vary greatly in size, ranging from a cubic yard to a height reaching five floors, and are built in churches, synagogues, concert halls, and homes. Small organs are called positive (easily placed in different locations) or portative (small enough to carry while playing). Increasingly hybrid organs are appearing in which pipes are augmented with electronic additions. Great economies of space as well as cost are possible especially when the lowest (and largest) of the pipes can be replaced.