ob·bli·ga·to /ˌɒblɪˈgɑtoʊ; It. ˌɔbbliˈgɑtɔ/ adjective, noun, plural -tos, -ti  /-ti/ Music.

1. (used as a musical direction) obligatory or indispensable; so important that it cannot be omitted.

2. an obbligato part or accompaniment.
3. a continuing or persistent subordinate or background motif.
4. a subordinate part of a solo.

Also, obligato.

1715–25; < It: bound, obliged < L obligātus; see obligate

Source: Dictionary.com.

I hear a whisper of their music: a jingling, a tinkle, to a rather plaintive obbligato of pipes.

Source: Tales of the Dying Earth by Jack Vance.