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diphthong /ˈdifˌTHäNG/ /ˈdipˌTHäNG/

Noun. Late Middle English: from French diphtongue, via late Latin from Greek diphthongos, from di- 'twice' + phthongos 'voice, sound'. A sound formed by the combination of two vowels in a single syllable, in which the sound begins as one vowel and moves toward another. -- Oxford English Dictionary