coauthor vs writer

coauthor and writer both are nouns.

Word	Noun	Adjective	Verb	Adverb
coauthor	Yes	No	No	No
writer	Yes	No	No	No

As nouns, writer is a hypernym of coauthor; that is, writer is a word with a broader meaning than coauthor:

coauthor: a writer who collaborates with others in writing something
writer: writes (books or stories or articles or the like) professionally (for pay)

Other hypernyms of coauthor include author.

coauthor (noun)	writer (noun)
a writer who collaborates with others in writing something	writes (books or stories or articles or the like) professionally (for pay)
	a person who is able to write and has written something