Melisma

[Couch Potatoe]

A piece of melody; a song or tune, -- as opposed to
recitative or musical declamation.

[Marquetta]

Singing more than one note on a single syllable. To be used in moderation. Abused to the point of absurdity by RnB and pop singers (usually but not always female) as a way of showing off vocal capabilities, much like an Yngwie Malmsteen solo. Egregious offenders include Mariah Carey, Celine Dion, Christina Aguilera, and Boyz II Men.

Sometimes called runs, or vocal runs.