Definition - Two graphemes that are used together to represent one phoneme (i.e. two letters are used to represent one sound).

Example -
The sh in ship is a digraph.

Etymology -
The word was coined by combining the Greek di, twice, with the Greek graphe, writing.

Oxford English Dictionary -
Its first citation is from 1788:
"All improper diphthongs, or, as I have called them, digraphs, are changed into the single vowels which they stand for."
(T. Sheridan (L.))

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