A libretto is the text used in, or intended for, an extended musical work such as an opera, operetta, masque, oratorio, cantata, or musical. The term "libretto" is also sometimes used to refer to the text of major liturgical works, such as Mass, requiem, and sacred cantata, or the story line of a ballet.

Libretto (pl. libretti), from Italian, is the diminutive of the word "libro" (book). A libretto is distinct from a synopsis or scenario of the plot, in that the libretto contains all the words and stage directions, while a synopsis summarizes the plot. However, some ballet historians use the word libretto to refer to the 15–40 page books which were on sale to 19th century ballet audiences in Paris and contained a very detailed description of the ballet's story, scene by scene.

The relationship of the librettist (that is, the writer of a libretto) to the composer in the creation of a musical work has varied over the centuries, as have the sources and the writing techniques employed.