Here is a list of some technical terms used on the website with short explanations. At the moment, the links open in a new window, so you will need to click the "back" button on your browser.

A short list of organ terminology
Bass Flute The 8-foot stop on the pedals which gives some definition to the pedal section.
Bourdon The 16-foot large-scale stop on the pedals on which the bass line is played.
Console The framework containing the manuals, the pedals and everything else the organist needs to play the organ.
Diapason The word usually used in English to indicate the foundation or basic stop of the organ. In German organ terminology, this stop is often called "Prinzipal", which is more descriptive of its function. The English term actually comes from Greek διαπασων "the concord of the first and last notes of the scale, an octave" (apologies to Greek scholars, this site doesn't cater easily for accents in Greek). It is often called "open" because the end of the pipe is just that — open, rather than fitted with a stopper (see "Gedackt"). The Horsley organ's diapason pipes are attractively stencilled and are placed at the front of the organ case.
Dulciana A small-scale labial pipe usually (as in the Horsley organ) of 8-foot pitch. According to Audsley, it was introduced by John Snetzler and has been a great favourite of English organ builders.
Gemshorn An open labial stop of 4-foot pitch, the pipes of which are conical in form. This has the effect of suppressing some of the harmonics to produce a clear, penetrating note.
Great One of the manual divisions of the organ, usually containing the "main" section of the organ, and usually called Hauptwerk in German. It is controlled from a manual, naturally called the Great, usually placed below the Swell and above the Choir in the organ console.
Lieblich Bourdon A 16-foot bourdon on the swell of a smaller scale than that on the pedals, which therefore produces a softer, more flutey, tone. That is the reason for the designation lieblich, which could be rendered "lovely" or "pleasant". This was certainly the intention of the builders.
Lieblich Gedackt A soft 8-foot stop on the swell, which provides a contrast to the flute on the Great. Again, this is the reason for the designation lieblich, which could be rendered "lovely" or "pleasant". The term gedackt means "covered", and is not (as Audsley writes) an incorrect form, but merely a variant.
Manual Essentially, the organ keyboard, played with the hands (Latin manualiter), hence the name. The Horsley organ has two manuals, the Great and the Swell.
Mixture A compound stop, which in the Horsley organ contains two ranks of pipes (other organs may contain more) of high pitch, used with stops of 8-foot pitch to produce a bright tonal colour.
Oboe An reed stop on the organ characterized by its similarity to the orchestral oboe. Click here to play a C-Major scale on an oboe.
Pedal The division of the organ operated by the feet. In Bach's necrology, it is recorded that he was able to play with his two feet such passages as would have made a not unskilled keyboard player using all five fingers despair. Most of us can only aspire to this blessed state. The pedal pipes are usually the largest in the organ. The Horsley organ has one 16-foot pedal bourdon, and an 8-foot borrowed from the Swell. Some organs have even larger pedal pipes.
Piccolo A 2-foot flute stop, rather like the orchestral piccolo. On the Horsley organ, this is often played with the 8-foot diapason and the 4-foot principal to form the chorus on the great, which produces the characteristic organ sound.
Pitch Pitch refers to the perceived frequency of a sound: the higher the frequency, the higher the pitch; conversely, the lower the frequency, the lower the pitch. When pipes of different pitches are sounded, a so-called chorus is built up and the characteristic sound many people associate with an organ is produced.
Portative A small pipe-organ which can be carried around (from Latin portare "to carry"). Often, these organs were played resting on the player's left knee. The left hand would then activate the bellows and the right hand would play the keys. Of course, the keyboard would be of a limited compass, or else the organ would be too heavy. See the Wikipedia article for further explanation and some pictures.
Positive A small pipe-organ which, in contrast to the portative, is "put" in one place (hence the name from Latin ponere "to put"). These usually have a single manual and may have one or more ranks of pipes. See the Wikipedia entry for a fuller explanation and some pictures.
Principal In the Horsley organ, a 4-foot pipe which goes with the diapason to form the basic organ chorus. It is called "Principal" because the "Middle-C" of this rank of pipes is the one against which all the others are tuned (it has no tuning slide).
Rank A set of organ pipes which is brought into play by drawing one of the stops. Each pipe in a rank corresponds to one of the keys on the keyboard, or one of the pedals on the pedalboard. Most keyboards have 61 or 62 notes, so there would be 61 or 62 pipes in a rank; most pedalboards have 30 or 32 notes, so there will be 30 or 32 notes in a pedal rank. In older organs, the pedals are sometimes of smaller compass. The Horsley organ has 58 keys on each manual and 30 pedals.
Reed A type of organ pipe which uses a thin strip or tongue of metal (that's why they're sometimes called lingual pipes). Air enters through the bore and makes the reed vibrate in a similar fashion to an oboe. The resulting sound is rich in harmonics and the resonator above the base of the pipe (called the boot) accentuates the required note.
Rohr Flute The equivalent of the German Rohrflöte, an 8-foot half-covered stop which has a pleasant flutey — liquid, bright and singing — tone, rather like the orchestral flute. Rohr is just the German word for a tube or pipe.
Scale A sequence of notes ordered by relative frquency. Click here to play the C Major scale on an 8-foot Principal stop. The scale starts on Middle C and the subsequent notes are D (9/8), E (5/4), F (4/3), G (3/2), A (5/3), B (15/8) and then the C (2/1) above Middle C before descending again and ending up on Middle C. If Middle C is 264 Hz, the C above Middle C is 528 Hz, and the other frequencies may be worked out using the ratios in brackets.
Scaling The sum of the measurements of an organ pipe, especially the ratio of the length to the circumference.
Stop A rank of pipes on the organ, engaged by the sliders on the top of the wind chest. Also used to indicate the stop-knobs which are pulled to engage individual ranks of pipes — hence the term "pulling out all the stops".
Swell One of the manual divisions of the organ, so called because it is installed in an enclosed box (swell box) with louvres to allow the sound be increased when the louvres are open and decreased when they are closed. (German: Schwellwerk, but note that German organs did not typically have enclosed divisions, so this might correspond to the Oberwerk, literally "upper work".)
Tremulant A device to vary the wind pressure on the swell so as to produce a tremulous note.
Viol d'Orchestre An 8-foot stop intended to imitate the sound of the orchestral viola. It is of small scale (and, spookily, was introduced by the English organ-builder William Thynne) and prodcues a quiet, reedy tone..
Voix Celeste An 8-foot stop on the swell which it tuned slightly sharp and therefore produces a tremulous tone redolent of a "celestial voice"(or what one might imagine a heavenly voice to sound like).
Wald Flute The equivalent of the German Waldflöte "sylvan flute". In the Horsley organ, this is a wooden stop of 4-foot pitch.
2-foot pitch 2-foot pipes sound two octaves above normal piano pitch, so a Middle-C (or C5) would sound at 1045.5 Hz instead of 261.6 Hz. Click here to play the C-Major scale on a 2-foot Superoctave stop.
4-foot pitch 4-foot pipes sound one octave above normal piano pitch, so a "Middle-C" (or C5) would sound at 523.3 Hz instead of 261.6 Hz. Click here to play the C-Major scale on a 4-foot Octave stop.
8-foot pitch Corresponds to normal pitch on a piano (i.e. Middle-C is ca. 261.6 Hz). The pipes are named from the nominal length of the lowest pipe of the rank (in other words, the C two octaves below Middle-C, or C2). Click here to play the C Major scale on an 8-foot Principal stop.
16-foot pitch 16-foot pipes sound one octave below the corresponding 8-foot pipes when the same key is depressed (i.e. Middle-C would be 130.8 Hz instead of 261.6 Hz).