diphthong
Concise Oxford English Dictionary © 2008 Oxford University Press:
diphthong /ˈdɪfθɒŋ, ˈdɪp-/
▶noun a sound formed by the combination of two vowels in a single syllable, in which the sound begins as one vowel and moves towards another (as in coin).
■ a digraph representing the sound of a diphthong or single vowel (as in feat).
■ a ligature (such as æ).
– derivatives
diphthongal /-ˈθɒŋg(ə)l/ adjective,
diphthongization (or diphthongisation) noun,
diphthongize (or diphthongise) verb.
diphthongal /-ˈθɒŋg(ə)l/ adjective,
diphthongization (or diphthongisation) noun,
diphthongize (or diphthongise) verb.
– origin ME: from Fr. diphtongue, via late L. from Gk diphthongos, from di- ‘twice’ + phthongos ‘sound’.
'diphthong' also found in these Oxford entries:

