Obbligato
/ˌɑːblɪˈɡɑːtəʊ/
Definition 1 of 1
Pro Tip 1/3Music Specific
The term is most often used in musical contexts to refer to a necessary instrumental part. 
The cellist practiced her obbligato thoroughly for the quartet.
Pro Tip 2/3Not Interchangeable
Don't confuse obbligato with a solo; obbligato supports the main melody, not always the spotlight. 
The horn's obbligato complemented the singer's melody.
Pro Tip 3/3Harmonic Foundation
An obbligato part usually weaves into the harmony, adding depth to the piece. 
His piano obbligato provided a rich foundation for the piece.