Triphthong
Pronunciation : Triph"thong
Part of Speech : n.
Etymology : [Pref. tri- + -phthong, as in diphthong: cf. F. triphthonque.] (Ortho?py)
Definition : Defn: 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.
Source : Webster's Unabridged Dictionary, 1913