Overture

Instrumental introduction to an opera, ballet, or oratorio

Overture (from French ouverture, lit. "opening") is a music instrumental introduction to a ballet, opera, or oratorio in the 17th century. During the early Romantic era, composers such as Beethoven and Mendelssohn composed overtures which were independent, self-existing, instrumental, programmatic works that foreshadowed genres such as the symphonic poem.

From Wikipedia, under CC BY-SA. More on occurri.