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.
tri- + (di)phthong.(American Heritage® Dictionary of the English Language, Fourth Edition)
From Ancient Greek τρία (tria, "three") + φθόγγος (phthongos, "sound"). (Wiktionary)