Phonetics. an unsegmentable, gliding speech sound varying continuously in phonetic quality but held to be a single sound or phoneme and identified by its apparent beginning and ending sound, as the oi- sound of toy or boil.
a digraph, as the ea of meat.
a ligature, as æ.
verb (used with object), verb (used without object)
The production of a diphthong acoustically is a glide between two sounds.
British Dictionary definitions for diphthong
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)
a digraph or ligature representing a composite vowel such as this, as ae in Caesar
C15: from Late Latin diphthongus, from Greek diphthongos, from di-1 + phthongos sound
late 15c., from Middle French diphthongue, from Late Latin diphthongus, from Greek diphthongos "having two sounds," from di- "double" (see di- (1)) + phthongos "sound, voice," related to phthengesthai "utter, speak loudly."