That's it!

'dipthong' - no smart arse comments....

"A complex speech sound or glide that begins with one vowel and gradually changes to another vowel within the same syllable"

"the sound formed by two merged vowels, highly prevalent in English, eg the vowel sounds of 'loud', 'new', 'why' "

"a vowel sound that starts near the articulatory position for one vowel and moves toward the position for another"

I'm not going to quote the entire volume...

Damn Senelity!