digraph vs digram

digraph and digram both are nouns.

Word	Noun	Adjective	Verb	Adverb
digraph	Yes	No	No	No
digram	Yes	No	No	No

As nouns, digraph and digram are synonyms defined as:

digraph and digram: two successive letters (especially two letters used to represent a single sound: `sh' in `shoe')

digraph (noun)	digram (noun)
two successive letters (especially two letters used to represent a single sound: `sh' in `shoe')	two successive letters (especially two letters used to represent a single sound: `sh' in `shoe')