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
"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 diphthong retains the two parts in its Greek ancestor diphthongos: 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/.