Diphthong

Diph″thong (?; 115, 277), n. [L. diphthongus, Gr. �; δι- = δίσ- twice + � voice, sound, fr. � to utter a sound: cf. F. diphthongue.] (Orthoëpy) (a) A coalition or union of two vowel sounds pronounced in one syllable; as, ou in out, oi in noise; — called a proper diphthong. (b) 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.