Some people use "librettist" as "lyricist" for every classical work (including art songs). Some people use it for opera and the like only, but on all works too. Some people use it only on the main opera / musical / whatever work and use "lyricist" for specific arias, etc. The latter approach seems better to me, especially since librettist doesn't always equal lyrics writer (especially for musicals) but the whole thing needs some discussion.