• Webster's Revised Unabridged Dictionary
    • n Triphthong (Orthoëpy) 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 .
    • ***
Century Dictionary and Cyclopedia
    • n triphthong A combination of three vowels in a single syllable forming a simple or compound sound; a group of three vowel characters representing combinedly a single or monosyllabic sound, as eau in beau, ieu in adieu, eye, etc.; a vowel trigraph.
    • ***
Chambers's Twentieth Century Dictionary
    • n Triphthong trif′thong a combination of three vowels to form one sound
    • ***


Webster's Revised Unabridged Dictionary
Pref. tri-, + -phthong, as in diphthong,: cf. F. triphthonque,
Chambers's Twentieth Century Dictionary
Gr. treis, three, phthongos, sound.


In literature:

A TRIPHTHONG is produced by running together three vowel sounds in the same syllable.
"Orthography" by Elmer W. Cavins
The Serbian language being phonetic does not employ double consonants, diphthongs or triphthongs.
"Hero Tales and Legends of the Serbians" by Woislav M. Petrovitch