overture
formOH-ver-chur
An orchestral introduction to an opera, oratorio, or suite.
An overture is an orchestral introduction to an opera, ballet, oratorio, or suite. In the opera house, the overture serves multiple purposes: it settles the audience, establishes the mood, and often previews the main themes that will appear throughout the work.
The French overture (slow-fast-slow) and the Italian overture (fast-slow-fast) were the two main Baroque types. By the Classical era, the overture had become a single fast movement, often in sonata form. Some overtures — like Rossini's William Tell and Beethoven's Leonore No. 3 — became so popular that they entered the concert repertoire as standalone works.
Rossini was so notoriously lazy about writing overtures that he sometimes recycled them between operas. The overture to The Barber of Seville was originally written for a completely different opera.