[ob.bli.ga.to] adj [It, obligatory, fr. pp. of obbligare to oblige, fr. L obligare--more at oblige] (1794): not to be omitted: obligatory--used as a direction in music--compare ad libitum
[2]obbligato n, pl -tos also -ti (1845) 1: an elaborate esp. melodic part accompanying a solo or principal melody and usu. played by a single instrument "a song with violin ~"
2: accompaniment 2b; esp: an attendant background sound