A technical word in linguistics. It simply means two letters written together to represent one sound. Examples are '-th-', '-ph-' and '-gh-'. The last shows that a digraph does not necessarily have a constant value. The single digraph '-gh-' can represent at least eight sounds in English.
A particular case is the ligature digraph, commonly known to printers simply as a ligature, where two letters are written in a single pen-stroke, or printed with a single unit of type, or treated as one symbol on a computer, as with æ, œ and ß (the German double '-s-'. See further Obsolete letters). (& also began as such a digraph.)