Bible Dictionaries
Trigraph

Webster's Dictionary

(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.

Bibliography Information
Webster, Noah. Entry for 'Trigraph'. Noah Webster's American Dictionary. https://www.studylight.org/​dictionaries/​eng/​web/​t/trigraph.html. 1828.