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. 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.