Diph'thong1, n.
(dĭf/thong, or dĭp/thong) (Synop., § 130), n. [Gr. διφθόγγος, from δί, for δίς, twice, double, and φθόγγος, voice, sound, from φθέγγεσθαι, to utter a sound; Lat. diphthongus, Fr. diphthongue, Pr. diptongue, Sp. diptongo, Pg. diphtongo, It. dittongo.] (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.