Jubilus
Jubilus (plural jubili) is the term for the long melisma placed on the final syllable of the Alleluia as it is sung in the Gregorian chant. The structure of the Alleluia is such that the cantor first sings the word "alleluia," without the jubilus, and then the choir repeats the word with the melisma added. It is traditionally repeated at the end of the chant as well, although it was frequently omitted in the Middle Ages and is still omitted when the Alleluia is followed by a Sequence.