Noun
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 dictionaryFeatures rather more confined to Lincolnshire include: *Elaboration of standard English /eɪ/ or /iː/ into a complex triphthong approximating, and often transcribed -air- or -yair-. Source: Internet
Some linguists use the terms diphthong and triphthong only in this phonemic sense. Source: Internet
Syllabic structure A syllable requires a vowel (or diphthong or triphthong) to appear in the middle. Source: Internet
There are different possible realisations of these items: in slow, careful speech they may be pronounced as a two-syllable triphthong with three distinct vowel qualities in succession, or as a monosyllabic triphthong. Source: Internet