bigram vs written word

bigram and written word both are nouns.

Word	Noun	Adjective	Verb	Adverb
bigram	Yes	No	No	No
written word	Yes	No	No	No

As nouns, written word is a hypernym of bigram; that is, written word is a word with a broader meaning than bigram:

bigram: a word that is written with two letters in an alphabetic writing system
written word: the written form of a word

bigram (noun)	written word (noun)
a word that is written with two letters in an alphabetic writing system	the written form of a word