1. diphthong - Noun
2. diphthong - Verb
A coalition or union of two vowel sounds pronounced in one syllable; as, ou in out, oi in noise; -- called a proper diphthong.
A vowel digraph; a union of two vowels in the same syllable, only one of them being sounded; as, ai in rain, eo in people; -- called an improper diphthong.
To form or pronounce as a diphthong; diphthongize.
Source: Webster's dictionaryAn early change that separated Runic Danish from the other dialects of Old East Norse was the change of the diphthong æi to the monophthong é, as in stæinn to sténn "stone". Source: Internet
Among the phonological processes operating in Finnish dialects are diphthongization and diphthong reduction. Source: Internet
For example, the diphthong ai in aigu, þair and waita was not retroactively umlauted to ei as in e.g. Old Icelandic eigu, þeir and veita. Source: Internet
A third, rare type of diphthong that is neither opening nor closing is height-harmonic diphthongs, with both elements at the same vowel height. Source: Internet
For example, the Vai syllabary originally had separate glyphs for syllables ending in a coda (doŋ), a long vowel (soo), or a diphthong (bai), though not enough glyphs to distinguish all CV combinations (some distinctions were ignored). Source: Internet
Almost regularly, a plain iota is replaced by the epsilon-iota diphthong (commonly if imprecisely known as itacism ), e.g. ΔΑΥΕΙΔ instead οf ΔΑΥΙΔ, ΠΕΙΛΑΤΟΣ instead of ΠΙΛΑΤΟΣ, ΦΑΡΕΙΣΑΙΟΙ instead of ΦΑΡΙΣΑΙΟΙ, etc. Jongkind, Dirk (2007). Source: Internet