what is a diphong?

Answer 1

it is a vowel used to replace a vowel and constanant
for ex. down, using a diphong d-AH-n
but it still sounds the same while you are singing

Answer 2

If you diphthong --

http://dictionary.reference.com/browse/diphthong

Answer 3

http://www.youtube.com/watch?v=f4Q1QN-SFCM