Diphthong Definition

dĭfthông, -thŏng, dĭp-

diphthongs

noun

A complex vowel sound made by gliding continuously from the position for one vowel to that for another within the same syllable, as ( = ä→) in down, (ī = ä→ē) in ride, ( = ô→ē) in boy.

Webster's New World

Either of the ligatures æ or œ, pronounced as diphthongs in classical Latin.

Webster's New World

(phonetics) A complex vowel sound that begins with the sound of one vowel and ends with the sound of another vowel, in the same syllable.

For example: "ae", "au", "ou"

Wiktionary

Other Word Forms of Diphthong

Noun

Singular:

diphthong

Plural:

diphthongs

Origin of Diphthong

Middle English diptonge from Old French diptongue from Late Latin diphthongus from Greek diphthongos di- two di–¹ phthongos sound

From American Heritage Dictionary of the English Language, 5th Edition
French diphtongue, from Ancient Greek δίφθογγος (diphthongos, “two sounds”), from δίς (dis, “twice”) + φθόγγος (phthongos, “sound”).

From Wiktionary