(trĭf′thông′, -thŏng′, trĭp′-)
A compound vowel sound resulting from the succession of three simple ones and functioning as a unit.

triph·thon′gal (-thông′əl, -thŏng′əl) adj.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
References in periodicals archive ?
As a result, English words whose triphthongal vowels are /aie/ as in 'fire' and 'tired', leial as in 'layer' and loial as in 'loyal' and 'royal' are pronounced as [faja], [tajad], [leja], [lojal] and [rojal] in the spoken English of L1 Shona speakers.
Triphthongs may also be involved at word junctures (gente, aiuto!), or triphthongal fonosillabe (like the Susanna example) may be created as a result of junctures.(12) Syllabification of connected speech differs from lexical syllabification to the extent that word junctures combine to reduce the number of syllables.
Any compound of ea formed up to that time would contain a u; and in a compound [Donuea.sup.*] with a triphthongal sequence it would naturally be the middle vowel that with the passage of time was simplified out of existence.