Definition - A sound formed by contracting two vowel sounds into one syllable, e.g. the vowel sounds in the words coin, boil, and about.

1. The process of changing a pure vowel into a diphthong is called diphthongization.

Etymology -
The word ultimately derives from the Greek diphthongos, double sound (from di, double + phthongos, sound or voice).

Oxford English Dictionary -
Its first citation is from 1483:
"A Diptonge [MS. A. Dypton], diptongus."
( Cath. Angl. 100/2)

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