/ obli'gahtoh/ adj said of a musical part or accompaniment: not to be omitted.
[Italian obbligato obligatory, past part. of obbligare to oblige, from Latin obligare]
*Definition from The New Penguin English Dictionary
ob-bli-ga-to (ob-li-ga-to) adjective - not to be left out; obligatory
(used of a musical part or accompaniment that is considered prominent but usually ornamental).