Trigraph (orthography)

A trigraph (from Ancient Greek τρεῖς (treîs) 'three', and γράφω (gráphō) 'to write')

digraph (from Ancient Greek δίς (dís) 'double', and γράφω (gráphō) 'to write, draw, paint, etc.')) is a group of three characters used to represent a single sound or a combination of sounds that does not correspond to the written letters combined.