diph ‧ thong /ˈdɪfθɒŋ, ˈdɪp- $ -θɒːŋ/ BrE AmE noun [countable] technical
[ Date: 1400-1500 ; Language: French ; Origin: Late Latin diphthongus 'two sounds' , from Greek phthongos 'voice, sound' ]
a vowel sound made by pronouncing two vowels quickly one after the other. For example, the vowel sound in ‘main’ is a diphthong.