from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A compound vowel sound resulting from the succession of three simple ones and functioning as a unit.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A monosyllabic vowel combination usually involving a quick but smooth movement from one vowel to another that passes over a third one.
from the GNU version of the Collaborative International Dictionary of English
- n. A combination of three vowel sounds in a single syllable, forming a simple or compound sound; also, a union of three vowel characters, representing together a single sound; a trigraph.
from The Century Dictionary and Cyclopedia
- n. A combination of three vowels in a single syllable forming a simple or compound sound; a group of three vowel characters representing combinedly a single or monosyllabic sound, as eau in beau, ieu in adieu, eye, etc.; a vowel trigraph.
A triphthong is a sequence of three vowels making up one syllable.
A triphthong is the union of _three_ vowels pronounced in like manner; as, _eau_ in beau, _iew_ in view.
The union of three vowels in one sound is called a triphthong; as iew in view.
The Standard Speller; Containing Exercises for Oral Spelling; also, Sentences for Silent Spelling by Writing from Dictation. In Which the Representative Words and the Anomalous Words of the English Language are so Classified as to Indicate Their Pronunciation, and to be Fixed in the Memory by Association.
However, for purposes of graphic accenting, any sequence made up of an open vowel between two closed atonic vowels will always be considered as a triphthong, regardless of its actual articulation in one or two syllables.
A sequence of atonic closed vowel + open vowel + atonic closed vowel can be pronounced, in certain cases, as one syllable, a triphthong, and in others as two distinct syllables, with a hiatus followed by a diphthong, or vice versa.
For a triphthong to exist, two closed atonic vowels (“i” or “u”) and, between them, an open vowel (“a”, “e”, or “o”): anunciáis, guau, miau, confiéis.
A triphthong is PROPER if all three of the vowels are sounded.
A triphthong is IMPROPER or is called a TRIGRAPH if one or two of the three vowels is silent; as in ad_ieu_, b_eau_ty.
Only the first two of these four words contain a triphthong.
Within a word two or three contiguous vowels usually combine to form a diphthong or a triphthong respectively