The organ (from Greek όργανον - organon, "organ, instrument, tool") is a keyboard instrument played using one or more manuals and a pedalboard. It uses wind moving through metal or wood pipes and/or it uses sampled organ sounds or oscillators to produce sound, which remains constant while a key is depressed. Its sounds, which vary widely in timbre and volume, are divided according to ranks and controlled by the use of stops. The keyboard is not expressive and does not affect dynamics. Organs vary greatly in size, ranging from a cubic yard to a height reaching five floors , and are located primarily in churches, concert halls, and homes. The organ is one of the oldest musical instruments in the Western musical tradition, and carries a rich history connected with Christian liturgy and civic ceremony.

The term "organ" may be applied to a variety of instruments which do not have all of the traits listed above. The most well-known type of organ is the pipe organ and is used in many church services and classical music concerts. Another prevalent type is the electronic organ, which does not have pipes and generates its electronically-produced sound through one or more loudspeakers; these are often intended to be replacements for pipe organs but are also performed in genres ranging from rock to jazz. There are combinations of pipe and electronics used together to create electronic-pipe organs. In addition there are many other instruments that also may be considered organs, and these are used in many different ways. Organs are performed upon by organists and are built and maintained by organ builders.