Libretto is the textual script that forms the basis of an opera.
The libretto is the literary text underlying an opera, oratorio, or musical. Historically, librettists were often as famous as composers. Pietro Metastasio's librettos were set to music over 800 times in the 18th century. The finest librettos — Da Ponte's for Mozart, Hofmannsthal's for Strauss — are works of dramatic literature in their own right.