Origin: L. Diphthongus, gr.; = twice – voice, sound, fr. To utter a sound: cf. F. Diphthongue a coalition or union of two vowel sounds pronounced in one syllable; as, ou in out, oi in noise; called a proper diphthong.
a vowel digraph; a union of two vowels in the same syllable, only one of them being sounded; as, ai in rain, eo in people; called an improper diphthong.