flatmate vs friend

flatmate and friend both are nouns.

Word	Noun	Adjective	Verb	Adverb
flatmate	Yes	No	No	No
friend	Yes	No	No	No

As nouns, friend is a hypernym of flatmate; that is, friend is a word with a broader meaning than flatmate:

flatmate (noun)	friend (noun)
an associate who shares an apartment with you	a person you know well and regard with affection and trust
	a person with whom you are acquainted
	an associate who provides cooperation or assistance
	a person who backs a politician or a team etc.