The process of moving from one vowel sound to another is called gliding, and thus another name for diphthong is gliding vowel.
A sound change that turns a single vowel into a diphthong is called diphthongization.
EtymologyFrom the Greek, "two sounds"
Examples and Observations:
- "If you say the words hat and lip, you can hear that the vowel sound in each is singular in nature; that is, each contains only one kind of sound. But if you say the words out, bite, and toil, you will hear that the vowel sound of each, though restricted to one syllable, is composed of two different kinds of sound. These dual vowels are called diphthongs (literally, 'two voices' or 'two sounds'), as opposed to the singular vowels, which are monophthongs ('one voice' or 'one sound')."
(Thomas E. Murray, The Structure of English. Allyn and Bacon, 1995)
- "The vowel [a] is heard in eastern New England speech in ask, half, laugh, and path and in some varieties of Southern speech in bye, might, tired, and the like. It is intermediate between [ɑ] and [æ], and is usually the first element of a diphthong (that is, a two-vowel sequence pronounced as the core of a single syllable) as in right and rout."
(John Algeo and Thomas Pyles, The Origins and Development of the English Language, 5th ed. Thomson Wadsworth, 2005)
- "'New York tawk features a diphthongal aw sound,' [Charles H.] Elster observes, 'that in heavy New Yorkese sounds almost disyllabic.' ( . . . [L]et me translate. A diphthong is the gliding sound of combining vowels, as in the oy in the head-smacking Yiddish oy veh. Disyllabic means 'having two syllables.') 'It's impossible for me to transliterate this elongated aw here, but ask a dyed-in-the-wool New Yorker to pronounce dog and coffee and you'll come close."
(William Safire, "Yagoddaprollemwiddat?" The New York Times Magazine, September 17, 2000)