A 'digraph' means 'one symbol written as two characters'. In printing it meant two letters cast as one piece of lead, e.g. ae, fi, ...
In graph theory, 'digraph' is an abbreviation of 'directed graph'. You may want to use the more explicit tag directed-graph.
In computer programming, digraphs and trigraphs are sequences of two and three characters respectively, appearing in source code, which a programming language specification requires an implementation of that language to treat as if they were one other character.
Various reasons exist for using digraphs and trigraphs: keyboards may not have keys to cover the entire character set of the language, input of special characters may be difficult, text editors may reserve some characters for special use and so on. Trigraphs might also be used for some EBCDIC code pages that lack characters such as { and }.
