The organ is a type of keyboard musical instrument, distinctive because the sound is not produced by a percussion action, as on a piano or celesta, or by means of vibrating strings, as on the harpsichord.
Instead, organs produce sound by means of flowing air. Organs date back to medival times, when they originated as portable instruments used for accompaniment in choral music. As the instruments became larger, they were installed permanently in a fashion similar to the church organs of today.
The sound-producing elements in organs are generally reeds and flutelike pipes. Reeds --thin strips of metal fastened at one end with the rest allowed to vibrate freely-- are to be found in organs of all sizes. The flutelike pipes, which work using vibrating columns of air, are used on larger instruments.
Related instruments include:
Organs were the first keyboard instruments. They require much less technology to produce than the piano or the harpsichord.
The pipe organ is a common kind of organ, with churches often housing such an instrument - when the word "organ" is used on its own in a classical music context, the pipe organ is most often meant. It is this instrument that is sometimes called the "king of instruments" in that, when played by a capable perfomer, richer and more complex music can be produced than with any other single instrument.
The versatility of the organ is attributable to the builders' ability to attach any number of instruments, or 'voices', to the keyboards which can be selected individually or in multiples by the operator. A good organist can produce a complex symphony of sounds simply by selecting which voices are used by which keyboard.
Voices are selected by 'stops'. The colloquial phrase "to pull out all the stops" originates from the simultaneous use of the multiple voices of an organ to produce a fine sound. Much air is used to power an organ when all the stops are pulled out, and in days when there were no electric motors, the profligate use of air required much labor, and was used only for special occasions.
The word organ originates from the instrument's appearance, with many pipes protruding, bearing a passing resemblance to that of anatomical organs, perhaps the heart most especially.
There are also various electrically operated and electronic organs, such as the Hammond Organ. While the Hammond was of imitative intent, it has developed something of a cult following and is at its best when used to produce a sound of its own rather than an attempt at a pipe-organ-like sound.