Obbligato
From Conservapedia
Obbligato is an elaborate melodic part accompanying a solo or principal melody.
The obbligato instrument is necessary for the playing of the piece, unlike, say, a notated ground bass, or bass chords. It frequently plays nearly as big a part in the composition as the principals. The clarinet in Schubert's "Shepherd on the Rock" the viola in Brahms' two Opus 91 songs, the trumpet in Shostakovich's Second Piano concerto... they are "obliged" to be there.
