Definition:
two vowel sounds joined in one syllable to form one speech sound, e.g. the sounds of “ou” in out and of “oy” in boy
Example:
“A glitch refers to some piece of technology’s failure to do something it’s intended to do. But Siri is doing exactly what it was built to do…. Siri would have an actual glitch if it couldn’t understand diphthongs or something.” — Damon Poeter,PC Magazine, December 1, 2011
About the Word:
Evoking the dual nature of diphthongs themselves, the word diphthongretains the two parts in its Greek ancestor tdiphthongos: di- meaning “two” and phthongos meaning “sound” or “voice.”
The word is a bit strange-looking to English speakers, a fact reflected in the two pronunciations the word has, one with a first syllable of /dif/ and one with a first syllable of /dip/.
My note: phthongos sounds a bit like tongue… and that can mean language,as we know.I do know this word but had not looked at its origin.Those ancient Greeks gave us plenty to think about and with from Language to Logic and Mathematics… are we less intelligent or just too busy?
How my heart sings 
Had to lecture on these ‘Subtleties of Grammar’ in Linguistics. 😉 Delightful. Hugs!
S that is why you know so much!!xx
One never ceases learning… Have a reasonable ‘portfolio’… 😉
You certainly do