noun.
A compound vowel sound resulting from the succession of three simple ones and functioning as a unit.
The Century Dictionary and Cyclopedia
noun.
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.
the GNU version of the Collaborative International Dictionary of English
noun.
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.