What is "triphthong"

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

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

In phonetics, a triphthong ( or ) (from Greek τρίφθογγος, "triphthongos", literally "with three sounds," or "with three tones") is a monosyllabic vowel combination involving a quick but smooth movement of the articulator from one vowel quality to another that passes over a third. While "pure" vowels, or monophthongs, are said to have one target articulator position, diphthongs have two, and triphthongs three.