obbligato
formob-lih-GAH-toh
An essential instrumental part that accompanies a vocal or solo line, too important to omit.
Obbligato (Italian for obligatory) describes an instrumental line that is so integral to the musical fabric that it cannot be left out. In Baroque arias, an obbligato instrument — often oboe, flute, or violin — weaves around the vocal line in a duet-like texture, complementing and commenting on the singer's melody.
The term is sometimes confused with ad libitum (at liberty), which is its opposite — an optional part that can be omitted. Bach was a master of obbligato writing: the violin obbligato in the Erbarme dich aria from the St. Matthew Passion creates one of the most heartbreaking instrumental-vocal dialogues in all music.
In 18th-century London, opera audiences would specifically attend performances to hear famous instrumentalists play obbligato parts — the instrumental soloist was sometimes a bigger draw than the singer.