Definitions:

1. A musical term referring to an instrumental part that is essential and prominent in a piece of music, typically used in classical compositions and contrasts with optional or ad-libbed parts. NOUN
   • The violin played an intricate obbligato that complemented the soprano's voice beautifully.
2. Used metaphorically to describe something indispensable or mandatory in a particular situation, such as a key element of a task or project. NOUN
   • Her input during the project meetings became an obbligato element of our success.

Synonyms:

• accompaniment
• Backing
• obligato