In music, the organ (from Greek ὄργανον organon, "organ, instrument, tool") is a keyboard instrument of one or more divisions, each played with its own keyboard, played either with the hands or with the feet. The organ is a relatively old musical instrument, dating from the time of Ctesibius of Alexandria, who is credited with inventing the water organ. It was played throughout the Greek and Roman world, particularly during races and games. During the early medieval period it spread from the Byzantine Empire, where it continued to be used in secular and imperial court music, to Western Europe, where it gradually assumed a prominent place in the liturgy of the Catholic Church. Subsequently it re-emerged as a secular and recital instrument.

The organ repertoire is among the largest for any solo musical instrument. Because of the  organ's (or pipe organ's) prominence in worship in Western Europe from the Middle Ages on, a significant portion of organ repertoire is sacred in nature. The organ's suitability for improvisation by a single performer is well adapted to this liturgical role and has allowed many blind organists to achieve fame; it also accounts for the relatively late emergence of written compositions for the instrument in the Renaissance. Although instruments are still disallowed in most Eastern churches, organs have found their way into a few synagogues as well as secular venues were organ recitals take place.