A trigraph is a group of three letters used to represent a single sound. For example, in the word schilling, "sch" represents the 'sh' sound. In the word "night", "igh" represents the vowel (a diphthong). Other examples are "beautiful" (eau), "adieu" (ieu).

Languages other than English also use trigraphs.

Some languages use trigraphs to represent their native triphthongs when using plain Latin alphabet without diacritics.

See digraph for more explanations.

Longer "multigraphs" are also known. It is quite possible that the longest one is an "octagraph" schtschj used in German language to represent a Russian language palatalized phoneme щь (which is, by the way, represented by a digraph in Russian).