trigraph

n. group of three successive letters that make one sound

A trigraph (from the Greek words tria = three and grapho = write) is a group of three symbols, most commonly letters.Trigraphs are used in several fields, and for several purposes. Among them areIn computing:A C trigraph, a character representation in the C family of programming languages.As octal or decimal representation of byte values.Mnemonics for machine language instructions.There are text matching and data compression algorithms which act directly on trigraphs.In language:As language codes in ISO 639.A Trigraph (orthography), a sound representation in orthography.As abbreviations.In cryptography:As substitution group in a substitution cipher.As combinations in the Ling Qi Jing.
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(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.