Definition and background:
Pipe organ - A keyboard instrument that is sounded by air moving through pipes. The pipe organ usually has several manuals as well as pedals that control the air flow through combinations of pipes. A rank of pipes constitues one stop and consists of a row of pipes, one for each key on the keyboard. Organ pipes come in a variety of sizes, shapes, materials, and methods of setting the air columns to vibrate. The pipes can range in size from a quarter-inch in length to over 32 feet with the shorter pipes creating the higher pitches and the larger pipes creating the lowest pitches. Organ pipes can be any shape from round to square, conical to cylindrical, or from straight to curved. The differing shapes provide a variety of tone colors. The pipes are typically constructed from either wood or metal. These materials provide another factor in the tone color of the pipe. The pipes are broken into two categories for creating sound, flue pipes and reed pipes.The flue pipes produce sound on the same principle of a whistle or recorder. The reed pipes produce sound on the same principle as the clarinet. The organ is an ancient instrument popular since the Medieval era. The use of organs is most commonly associated with religious music. Theater Organ - A pipe organ housed in an ornate cabinet and console that has in addition to the usual organ pipes, other instruments that are controlled by the keyboard. Other instruments commonly added include stringed instruments, snare drum, bass drum, cymbals, bells, and other various percussion instruments. Theater organs were used in the early 20th century to accompany silent films and and to provide entertainment to the movie audience prior to the advent of sound films (1929, The Jazz Singer). The various additional instuments offered a wider range of sounds that would allow the player to create special sound effects to heighten the sense of reality. Although most theater organs have been destroyed or fallen into disrepair, a few examples still illustrate the fantastic sound of the era. Electric organ - A keyboard instrument that uses electronics to create sounds that closely resemble that of the pipe organ. The electric organ is used as a low-cost substitute for pipe organs and in popular music (i.e. rock, blues & jazz) as an alternate keyboard instrument sound. a wind instrument consisting of at least one row of pipes, which are made to sound by air being directed under pressure from a wind-raising device, and key-and pedal-boards, which admit air to the pipes by means of valves.
For more information on the music term "Organ" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary