The Collaborative International Dictionary of English v.0.48:

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.
       [1913 Webster]

WordNet (r) 3.0 (2006):

libretto
    n 1: the words of an opera or musical play