A libretto is the text a composer sets to music. It contains all the dialogue and stage directions of the play, and can be dramatic or comedic. Libretti (Italian plural for Libretto) without the music would simply be scripts.
The 'libretto' is the words or story of the opera.
In an opera, the libretto is the lyrics to all the music in the opera.
Libretto.
A person who writes the libretto for an opera is simply referred to as the librettist.
The lyrics of an opera are called the "libretto."