(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.
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.
or triphthongal fonosillabe (like the Susanna example) may be created as a result of junctures.
with a triphthongal sequence it would naturally be the middle vowel that with the passage of time was simplified out of existence.