Yin, I was taught this:
Singing different pitches in succession as part of the same syllable is a "melisma."
If you are changing vowels in succession, like singing a-e-i-o-u, that's a "diphthong."
With a melisma your mouth does not move but your pitches change. With a diphthong your mouth moves.
add a skeleton here at some point
about 4 hours ago