(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; as, eye, -ieu in adieu, -eau in beau, are examples of triphthongs.
TRIPHTHONG
Meaning of TRIPHTHONG in English
Webster's English dictionary. Английский словарь Webster. 2012