Webster’s Revised Unabridged Dictionary · 1913
Libretto
Li·bret′to (lĭ·brĕt′tō; It. lē·brāt′tō), n.; pl. E. Librettos (-tōz), It. Libretti (-tē). [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.