Definition:

In phonetics, a vowel in which there is a noticeable sound change within the same syllable. (In contrast, a single or simple vowel is known as a monophthong.) Adjective: diphthongal.

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.

See Examples and Observations, below. Also see:

• Accent and Accent Prejudice
• Digraph
• Phoneme
• Pronunciation
• Regional Dialect
• Speech

Etymology
From 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 [?] 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
  "'New York tawk features a diphthongalaw 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)
• The Birmingham (England) Accent
  "When you notice a regional accent, what you're chiefly noticing are the way people pronounce their long vowels (in words like see, saw, and sue) and diphthongs (in words like say, so, sow, soy, and sigh). 
  
  "English has quite a few of these, and the more an accent sounds them differently from the way you speak, the more difficulty you'll have in 'picking up' that accent. 
  
  "Some accents will be very close to your own, so you would have only a few features to learn. But Birmingham and Geordie are two that--compared with Received Pronunciation [RP]--have lots of really noticeable differences, such as these three:

  - The diphthong in words like float has a much more open onset, so that it sounds more like RP 'flout.'
  - The diphthong in words like nice begins with a back and rounded quality, so that it sounds like 'noice.'
  - The diphthong in words like loud begins with a front and higher quality, so that it sounds more like 'le-ood.'"

  (David Crystal and Ben Crystal, "Revealed: Why the Brummie Accent Is Loved Everywhere but Britain." Daily Mail, October 3, 2014)
• Canadian Raising
  "Canadian raising involves the diphthong spelled ou when followed by a voiceless consonant.  (A diphthong is a two-vowel combination functioning as a unit.) It’s used in words ending in t, like out, about, and pout, and in words ending with voiceless s, like house and mouse. (But not the verbto house, because that ends with a voiced z sound.) It’s called raising because the tongue is raised at the start of the diphthong. There’s no good way to show this with conventional spelling, but I’ll try: Instead of the usual 'ah-oo' of the diphthong, the tongue is raised to begin with so it comes out as 'uh-oo.' It’s often spelled oo.
  
  "This diphthong raising is widespread in Canada and has become known there, and in the United States as a marker of identity for Canadians."
  (Allan Metcalf, "O Canada! in New Orleans." The Chronicle of Higher Education, February 25, 2015)

Pronunciation: DIF-thong or DIP-thong

Also Known As: gliding vowel, compound vowel, complex vowel, moving vowels