diphthong
noun
diph·thong ˈdif-ˌthȯŋ 
ˈdip-
1
linguistics : a gliding (see glide entry 1 sense 4) monosyllabic speech sound (such as the vowel combination at the end of toy) that starts at or near the articulatory position for one vowel and moves to or toward the position of another
2
linguistics : digraph
3
: the ligature (see ligature sense 4) æ or œ
diphthongal
dif-ˈthȯŋ-(g)əl dip-
adjective dip- 
Example Sentences
The sounds of “ou” in “out” and of “oy” in “boy” are diphthongs.
Recent Examples on the Web Our heroes come from House Atreides, their elegance enhanced by that smooth classical diphthong. 
Nate Jones, Vulture, 28 Oct. 2021 The reader can almost hear Camilleri’s longtime translator, Stephen Sartarelli, chuckling over his rendition of Catarella’s chatter as that mishmash of h-dropping Cockney and diphthong-slaying Brooklynese. Washington Post, 8 Oct. 2021 Even the big diphthongs that my grandfather’s years in the States had tamped down swelled back to their former glory in this company. Kevin Dettmar, The New Yorker, 20 July 2019
Nate Jones, Vulture, 28 Oct. 2021 The reader can almost hear Camilleri’s longtime translator, Stephen Sartarelli, chuckling over his rendition of Catarella’s chatter as that mishmash of h-dropping Cockney and diphthong-slaying Brooklynese. Washington Post, 8 Oct. 2021 Even the big diphthongs that my grandfather’s years in the States had tamped down swelled back to their former glory in this company. Kevin Dettmar, The New Yorker, 20 July 2019 Word History
Etymology
Middle English diptonge, from Middle French diptongue, from Late Latin dipthongus, from Greek diphthongos, from di- + phthongos voice, sound
First Known Use
15th century, in the meaning defined at sense 1
Phrases Containing diphthong
Kids Definition
diphthong
noun
diph·thong ˈdif-ˌthȯŋ
ˈdip-
: two vowel sounds joined in one syllable to form one speech sound
The sounds of "ou" in "out" and of "oy" in "boy" are diphthongs.