Obbligato

obbligato (ŏbləgä`tō) [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. Misunderstanding of the term obbligato, however, resulted in a reversal of its meaning; when a violin part, for example, is added to a song it is called a violin obbligato, whereas it may be a superfluous ornament for which ad libitum would be a more precise direction.

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obbligato, obligato Music

1. not to be omitted in performance

2. an essential part in a score

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Obbligato 

an instrumental part in a musical work that must not be omitted; it is obligatory that it be performed. Obbligato parts can be of varying importance—ranging from those that are significant but still form part of the accompaniment to parts that share equal significance with the featured solo part. In 17th- and 18th-century operas, oratorios, and cantatas (for example, in Bach’s Mass in B Minor) one often encounters arias, and sometimes even duets, with the participation of an obbligato instrument (or several instruments) and the orchestra. The opposite of obbligato is ad libitum (“at will”).