A diphthong is a vowel in which the speaker's tongue changes position while it is being pronounced, so that the vowel sounds like a combination of two other vowels. The vowel sound in 'tail' is a diphthong.
diphthong in British English
(ˈdɪfθɒŋ, ˈdɪp-)
noun
1.
a vowel sound, occupying a single syllable, during the articulation of which the tongue moves from one position to another, causing a continual change in vowel quality, as in the pronunciation of a in English late, during which the tongue moves from the position of (e) towards (ɪ)
2.
a digraph or ligature representing a composite vowel such as this, as ou in mouth or æ in Cæsar
Derived forms
diphthongal (diphˈthongal)
adjective
Word origin
C15: from Late Latin diphthongus, from Greek diphthongos, from di-1 + phthongos sound
diphthong in American English
(ˈdɪfθɔŋ; often ˈdɪpθɔŋ)
noun
1. Phonetics
a complex vowel sound made by gliding continuously from the position for one vowel to that for another within the samesyllable, as ( (aʊ) ) in down; ( (aɪ) ) in ride; ( (ɔɪ) ) in boy; ( (oʊ) ) in boat
2. Printing
either of the ligatures æ or œ, pronounced as diphthongs in classical Latin