digraph vs alphabetic character

digraph and alphabetic character both are nouns.

Word	Noun	Adjective	Verb	Adverb
digraph	Yes	No	No	No
alphabetic character	Yes	No	No	No

As nouns, alphabetic character is a hypernym of digraph; that is, alphabetic character is a word with a broader meaning than digraph:

digraph: two successive letters (especially two letters used to represent a single sound: `sh' in `shoe')
alphabetic character: the conventional characters of the alphabet used to represent speech

Other hypernyms of digraph include letter, letter of the alphabet.

digraph (noun)	alphabetic character (noun)
two successive letters (especially two letters used to represent a single sound: `sh' in `shoe')	the conventional characters of the alphabet used to represent speech