(n.) Three letters united in pronunciation so as to have but one sound, or to form but one syllable, as -ieu in adieu; a triphthong.
Example Sentences:
Triphthong
Definition:
(n.) 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.