An organum is any one of a number of musical instruments which were the forerunners of the organ

Organ (music)

The organ , is a keyboard instrument of one or more divisions, each played with its own keyboard operated either with the hands or with the feet. The organ is a relatively old musical instrument in the Western musical tradition, dating from the time of Ctesibius of Alexandria who is credited with...

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The name comes from the Latin organum, meaning any tool in general or any musical instrument in particular (or an organ of the body), which in turn came from the Greek organon, with similar meanings, itself derived from ergon and so meaning something by which a task is accomplished. The name organum in turn gave rise to the modern everyday term organ.