digraph (n.)

A term used in PHONETICS/PHONOLOGY and GRAPHETICS/GRAPHOLOGY to refer to a GRAPHIC UNIT in which two symbols have combined to function as a single ELEMENT in a SYSTEM, e.g. [æ] for the vowel in RECEIVED PRONUNCIATION cat, or the linked æ or œ in the classical spelling of some English words (e.g. encyclopædia, onomatopœia).

In the study of reading and spelling, digraph refers to any sequence of two letters pronounced as a single sound. Examples include the first two letters of ship and the middle two letters of wool.