Diph"thong (?; 115, 277), n. [L. diphthongus, Gr. &?;; di- = di`s- 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.

Diph"thong, v. t. To form or pronounce as a diphthong; diphthongize. [R.]

• In phonetics, a complex vowel sound that begins as the sound of one vowel and ends as the sound of another vowel.
  
• joined vowels, pronounced as one, such as a and e in æ, and o and e in œ