A librettist is the person who writes the words of an opera, or sometimes of a similar long sung work. Those words as a whole are the libretto — a kind of script to be sung, not a novel and not the music. The composer writes the music; the librettist writes what the singers say and sing, including the story. In a pop song the closest everyday word is lyricist, but librettist is the special word for opera and that world. It is a rather fancy, specialist word; you meet it in music notes, theatre programmes, and serious reviews, not in the playground. It is built from libretto plus -ist, “a person who does that.” Libretto is Italian for “little book,” because the words used to be printed in a small book for the audience. Though actually the librettist is easy to forget: people name the composer and skip the person who wrote the words, the way film-goers name the director and forget the writer.