triphthong

A compound vowel sound resulting from the succession of three simple ones and functioning as a unit.

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 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.

A monosyllabic vowel combination usually involving a quick but smooth movement from one vowel to another that passes over a third one.