obbligato

(It., obligatory; Ger. obligato). Originally, an instrumental part that was essential in the performance, such as the many SONATAS for piano and another instrument obbligato. The meaning expanded to any instrumental or vocal part that is a concerted (and therefore an essential) part.

However, through some unexplained twist of fate, obbligato began to be used, particularly in popular arrangements, to indicate an optional part. For example, a song with cello obbligato would mean that such a part would be a desirable addition to the accompaniment, but not essential.