Definify.com

Webster 1913 Edition

Noun.

[Pref.

tri-

-graph

Three letters united in pronunciation so as to have but one sound, or to form but one syllable, as -ieu in adieu; a triphthong.

trigraph ‎(plural trigraphs)

(linguistics) A specific sequence of three letters, especially one used collectively to represent a single phoneme.

sequence of three letters