The pipe organ is a musical instrument commonly used in churches or cathedrals that produces sound by driving pressurized air (called wind) through pipes selected via a keyboard. Because each organ pipe produces a single pitch, the pipes are provided in sets called ranks, each of which has a common timbre and volume throughout the keyboard compass. Most organs have multiple ranks of pipes of differing timbre, pitch and loudness that the player can employ singly or in combination through the use of controls called stops.

A pipe organ has one or more keyboards (called manuals) played by the hands, and a pedalboard played by the feet, each of which has its own group of stops. The organ's continuous supply of wind allows it to sustain notes for as long as the corresponding keys are depressed, unlike the piano and harpsichord, the sounds of which begin to decay immediately after attack. The smallest portable pipe organs may have only one or two dozen pipes and one manual; the largest may have over 20,000 pipes and seven manuals.

The origins of the pipe organ can be traced back to the hydraulis in Ancient Greece in the 3rd century BC, in which the wind supply was created with water pressure. By the sixth or 7th century AD, bellows were used to supply organs with wind. Beginning in the 12th century, the organ began to evolve into a complex instrument capable of producing different timbres. By the 17th century, most of the sounds available on the modern classical organ had been developed. From that time, the pipe organ was the most complex man-made device, a distinction it retained until it was displaced by the telephone exchange in the late 19th century.