Overture

An introductory movement for orchestra intended to introduce an opera, oratorio, or other dramatic vocal composition by presenting themes to be heard later in the composition. Also, an independent composition for orchestra; in this case, it is called a " concert overture."an orchestral composition that introduces an opera, oratorio or similar work, and usually includes music that alludes to what is to follow. A concert overture, however, stands on its own and is not part of an opera or other work