Phonetics: A → vowel sound produced by a blended sequence of two separate vowels in a single syllable, where the sound begins as one vowel and moves toward another (as in loud, light, and lair).
From M.Fr. diphthongue, from L.L. diphthongus, from Gk. diphthongos "having two sounds," from → di- "double" + phthongos "sound, voice."