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

noun.
1. a persistent but subordinate motif
2. a part of the score that must be performed without change or omission

Beyond the Meaning

obbligato [Ital.,=obligatory], in music, originally a term by which a composer indicated that a certain part was indispensable to the music. Obbligato was thus the direct opposite to ad libitum [Lat.,=at will], which indicated that the part so marked was unessential and might be omitted.