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An instrumental introduction to an large work, like an oratorio or opera.
An overture is a fairly short piece of music used to introduce an opera. Overtures have also been written to be played as part of concerts.
The Light Cavalry Overture and the William Tell Overture are not at all the same piece. The Light Cavalry Overture (1865) was written by Franz von Suppé (1819-95) as the overture to one of his...
Prometheus. It's a symphony by Scriabin.