written word vs bigram

written word and bigram both are nouns.

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

As nouns, bigram is a hyponym of written word; that is, bigram is a word with a more specific, narrower meaning than written word:

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

Other hyponyms of written word include trigram, tetragram.

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