Obbligato

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); the spelling obligato is not acceptable in British English; however, it is often used as an alternative spelling in the US. 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).