trigraph

noun2

1. •
   A specific sequence of three letters, especially one used collectively to represent a single phoneme.

   • The letter “i” is pronounced as /aɪ/ when followed by the trigraph “ght”, as in right /raɪt/.
2. •
   A three-character sequence used to enter a single conceptual character.

   • These new features are charizing, stringization, token concatenation, string concatenation, trigraph replacement, and predefined macros. — 1993