Source : Webster's Revised Unabridged Dictionary (1913)

Libretto \Li*bret"to\ (l[i^]*br[e^]t"t[-o]; It.
   l[-e]*br[asl]t"t[-o]), n.; pl. E. {Librettos} (-t[=o]z), It.
   {Libretti} (-t[-e]). [It., dim. of libro book, L. liber. See
   {Libel}.] (Mus.)
   (a) A book containing the words of an opera or extended piece
       of music.
   (b) The words themselves.

Source : WordNet®

libretto
     n : the words of an opera or musical play
     [also: {libretti} (pl)]