A musical instrument played by one or more keyboards and often a set of foot pedals. The characteristic of organs is that they do not use strings and they have multiple voices selected by stops or their equivalent. The note also sounds for as long as the key is pressed.
The pipe organ is well known and seen in many churches employing pumped air and valves to direct the air to a series of long tuned pipes which are often visible. They come in many sizes and complexities.
Cinema Organs were pipe organs with additional effects such as percussion. Fairground organs were also pipe organs, usually automated like a Pianola with a music roll and with an animated diaplay usually of musicians. Gavioli is probably one of the best known.
The electronic organ is a very common instrument and by electronic effects can emulate a wide variety of organ sounds and other keyboard insruments such as Piano or Harpsichord. The Hammond developed in 1934 is probably the best known together with it's partner, the Leslie rotating horn speaker and today the Yamaha and Roland are ubiquitous.
Another type of organ is the Reed Organ, using air across reeds rather than through pipes. These are smaller than pipe organs and less complicated.