organ (instrument)
A class of keyboard instrument characterized by various sounds of unlimited sustain. The earliest common organs were pipe organs built for churches. Their sound came from pressurized air moving through specially formed metal pipes. Pipe organs are still in use in many Western churches and concert halls worldwide.
The pump organ (see Danbooru post #657165) is a small pipe organ conceived for home use. The air in this instrument is generated by foot-pedal bellows, which one pumps with alternating feet.
There are also electric organs (such as found at Danbooru post #519301) whose sounds are generated through circuits. The Hammond B3 is arguably the most famous of this type, widely used in jazz, blues and rock music.
A prominent feature of most types of organs is the existence of at least two manuals (keyboard sets; with a light, spring-weighted action unlike that of a piano), a pedalboard, and a number of "stops" or switches which activate different sets of pipes for varied color.
*Not to be confused with biological organs.*