What is a composition, for a full orchestra, designed as an introduction to an oratorio, opera, or ballet, or as an independent piece; -- called in the latter case a concert overture called?

overture — (Mus.) A composition, for a full orchestra, designed as an introduction to an oratorio, opera, or ballet, or as an independent piece; -- called in the latter case a concert overture. (Webster's 1913.)

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