Noun
a part of the score that must be performed without change or omission
a persistent but subordinate motif
Source: WordNetComposers such as Boccherini would publish sonatas for piano and obbligato instrument with an optional third movement—–in Boccherini's case, 28 cello sonatas. Source: Internet
Some of his chamber music involves trombones, as do many of his operas, used as an obbligato instrument. Source: Internet