ˈdīˌgraf, -aa(ə)f, -aif, -ȧf noun
Etymology: di- + -graph
1. : a group of two successive letters whose phonetic value is a single sound (as ea in bread, ng in thing ) or whose value is not the sum of a value borne by each in other occurrences (as ch in chin, where the value is t + sh)
2. : any group of two successive letters