noun
digraph
Linguistic term for a pair of letters representing a single sound, such as 'sh' in English or 'ch' in German.
英語の「sh」は二重音字の一例です。
The English 'sh' is an example of a digraph.
合字 (ligature) refers to two or more letters joined as a single glyph, while 二重音字 (digraph) is a pair of letters representing one sound.
Compound of 二重 (double) + 音字 (sound character), a calque from English 'digraph'.