obligato

n. requirement; obligatory musical accompaniment

In classical music obbligato usually describes a musical line that is in some way indispensable in performance. Its opposite is the marking ad libitum. It can also be used, more specifically, to indicate that a passage of music was to be played exactly as written, or only by the specified instrument, without changes or omissions. The word is borrowed from Italian (an adjective meaning fixed; from Latin obligatus p.p. of obligare, to oblige; in English the spelling obligato is also acceptable). The word can stand on its own, in English, as a noun, or appear as a modifier in a noun phrase (e.g. Organ obbligato).
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Noun
1. a persistent but subordinate motif
(synonym) obbligato
(hypernym) motif, motive
2. a part of the score that must be performed without change or omission
(synonym) obbligato
(hypernym) section, subdivision

n. obligato, obligatory musical accompaniment

n. obligato, requirement; obligatory musical accompaniment