Overture (French ouverture, meaning opening) in music is the instrumental introduction to a dramatic, choral or, occasionally, instrumental composition. During the early Romantic era, composers such as Beethoven and Mendelssohn began to use the term to refer to instrumental, programmatic works that presaged genres such as the symphonic poem.
History
17th century
The idea of an instrumental opening to opera existed during the 17th century. Peri's Euridice opens with a brief instrumental ritornello, and Monteverdi's L'Orfeo (1607) opens with a toccata, in this case a fanfare for muted trumpets. More important, however, was the Prologue, which comprised sung dialogue between allegorical characters which introduced the over-arching themes of the stories depicted.
French ouverture
As a musical form, however, the so-called "French overture" begins with the court ballet and operatic overtures of Jean-Baptiste Lully (Waterman and Anthony 2001), which he elaborated from a similar, two-s
Source:
http://en.wikipedia.org/wiki/Overture