libretto
Concise Oxford English Dictionary © 2008 Oxford University Press:
libretto /lɪˈbrɛtəʊ/
▶noun (pl. libretti /-ti/ or librettos) the text of an opera or other long vocal work.
– derivatives
librettist noun.
librettist noun.
– origin C18: from Ital., dimin. of libro ‘book’.
'libretto' also found in these Oxford entries:

