• Pipe organ
  A musical instrument played by one or more piano-type keyboards, where the sound is produced by air being blown into into one of more sets of pipes that are individually tuned to the notes required.
• Electronic organ
  A musical instrument played by one or more piano-type keyboards, where the sound is produced by electronic circuits and output through one or more loudspeakers.
• Pipe
  A hollow tube in which the air vibrates to create a sound of a definite pitch. The air in the tube can be made to vibrate either by means of a sharp edge across which air is blown, or by a vibrating reed. The tube can be almost any shape, cylindrical or conical, and does not even have to be a round cross-section. It must be open to the air at one end or both.

Superlatives
In the "Book of Musical Instrument Records", the organ should be mentioned more than any other instrument - it must have more superlatives attached to it than any other musical instrument. Mozart called it the "King of instruments".

• Size - the organ is by far the largest musical instrument. There are a few organs in the world with more than 20,000 pipes each (see the world's largest organs). The largest in the world is in the Atlantic City Convention Hall but this is not now fully working. The next largest is in a department store in Philadelphia. The largest one in Britain is the Anglican Cathedral in Liverpool, with nearly 10,000 pipes.
  What is the next largest musical instrument? - it depends what you count as a musical instrument - maybe a tower of bells? an alpine horn? or among more conventional instuments, perhaps a tuba?

• Volume - the organ must be the loudest musical instrument, unless you count electric amplification. When I played the Saint-Saens Organ Symphony at St. Mary's, Osterley with the Hounslow Symphony Orchestra in 2003, the maximum volume of the organ was about the same as the full fifty-piece orchestra (listen to this sample 152Kb MP3 file recorded at the concert). This organ works on probably a little over 2 inches of wind pressure, whereas the loudest stop in the world (at the Atlantic City Convention Hall) works on 100 inches!

• Skill required - this might be debatable, but more parts of the body are used when playing the organ than any other instrument. Two hands are used, often independently, both feet can be used independently of the hands, and then there are other things to be controlled (stops and pistons, for example) as well as page turns! The only other instrument that uses both hands and feet to any extent (apart from the busker with his "one-man band") is the harp - feet are used to change the pitch of strings. However, you could argue that more sensitive movements of the mouth etc. are required for woodwind and brass instruments.

• Volume range - the organ has the largest range of volume of a single instrument, again perhaps except for electronically amplified music. Even the loudest organ mentioned above also has stops that are so quiet that they can hardly be heard. Most other loud instruments (e.g. brass) can play quieter, but cannot play very quietly.

• Variations of sound - the most variation of sounds from the same instrument - this is subjective, but the organ was certainly the first instrument intended to imitate other instruments. In other words, it saved the need for a 50-piece orchestra. Modern electronic keyboards and synthesisers are the same, but these came along centuries after the organ. The medium-sized pipe organ can usually play a very convincing-souding trumpet and a reasonable flute, because the way the sound is produced in the organ is almost identical to the way the real instruments produce their sound. However, most other "imitations" are less convincing. In particular, the type of sound that is described as a "string tone" on the organ is in fact nothing like a stringed instrument, it is just a conventional description.

• Instrument variation - in most cases, no two pipe organs are the same - they are adapted to the building and needs of the instrument. Imagine going to buy a violin, say, and you are told that a consultant will come to your house to see what the acoustics are like before you are advised how big it should be, how many strings it should have or where you should play it. This is what should always happen with a pipe organ. There is no standard set of stops, and even a single named stop has no standard size or make of pipe.

• Pitch range - is greater than any other instrument. In physical terms, the organ I play can produces sounds ranging from from 32Hz (cycles a second) to 8192Hz. Large organs exceed this in both directions. The largest 64-foot pipes will produce a sound of only 8Hz. Electronic instruments may now be able to exceed the range, but that's rather artificial.

• Number of sound generators - the organ has more than any other instrument, and this is true even including modern electronic instruments. Each pipe in an organ is an individual sound generators, so a medium-sized organ has well over a thousand individual sound generators. Even the most modern electronic synthesisers and organs only have a maximum of 128 sound generator circuits (it is often quoted as maximum polyphony).

• Longest time to tune - the organ takes longer to tune than any other instrument, purely because there are more sound generators. e.g. a violin has 4 strings, so maybe takes 30 secs to tune. A standard piano has 88 notes, some with multiple strings, so has 200-300 sound generators, so takes a couple of hours to tune. An organ with many thousands of pipes can take many hours to tune.

• Most notes played (most of any instrument) - meaning the most sound generators used in a piece, or played at once. A single chord with say, six notes, might be using 8 pipes each (because 8 stops are pulled out), so 48 sound generators are involved, i.e. about the same as in a large orchestra. This is also the same as the number of puffs of wind, or number of signals (electric or pneumatic) required to play the chord. I once worked out that there are 5498 separate notes (key depressions) in Widor's Toccata (from his Symphony number 5), and since each note might play 20 pipes, that means a total of 110,000 signals or puffs of wind into sound generators. Even an ordinary hymn for a church service may be more than 5000 signals.

• Most mechanical complexity - before the Industrial Revolution, the organ was the most complex mechanism that anyone would ever come across. The next most complex would be a clock. In the middle ages, when organs first started to be used in churches, there were often built by the same person or company that built the church clock. In many ways in those days they performed a similar function to clocks and bells - perhaps to call people to worship, but not necessarily to take part in the service.