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

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "libretto":
   arrangement, book, continuity, copy, cue, draft, edition, hymnal,
   hymnbook, instrumental score, lines, lute tablature, music,
   music paper, music roll, musical notation, musical score, notation,
   opera, opera score, orchestral score, part, piano score, playbook,
   scenario, scene plot, score, script, sheet music, shooting script,
   short score, side, songbook, songster, tablature, text, transcript,
   transcription, version, vocal score, written music