obbligato

Not to be left out; indispensable. Used of an accompaniment that is an integral part of a piece.

An obbligato accompaniment.

In music, indispensable; so important that it cannot be omitted: especially used of accompaniments of independent value.

An accompaniment, whether for a solo or a concerted instrument, which is of independent importance; especially, an instrumental solo accompanying a vocal piece.

Also spelled obligato.

a part of the score that must be performed without change or omission

a persistent but subordinate motif