Triphthong \Triph"thong\, n. [Pref. tri- + -phthong, as in
diphthong: cf. F. triphthonque.] (Orthoepy)
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.
[1913 Webster]