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