Emily Dickinson Lexicon

DIPH'THONG, n. [Gr. διφθογγος; δις and φθογγος, sound; L. diphthongus.]

A coalition or union of two vowels pronounced in one syllable. In uttering a diphthong, both vowels are pronounced; the sound is not simple, but the two sounds are so blended as to be considered as forming one syllable, as in joy, noise, bound, out. [The pronunciation dipthong is vulgar.]