Definition -
(1) A sound formed by contracting three vowel sounds into one syllable (e.g. wire and tire).
(2) A word that has three consecutive vowels (e.g. beautiful).

Etymology -
The word derives the Greek phthongos, sound or voice.

Oxford English Dictionary -
Its first citation is from 1599:
"A triphthong is a sounding of three vowels into one syllable with one breath together…."
(Minsheu Span. Gram. (1623) 9 )

Please comment