coauthor vs joint author

coauthor and joint author both are nouns.

Word	Noun	Adjective	Verb	Adverb
coauthor	Yes	No	No	No
joint author	Yes	No	No	No

As nouns, coauthor and joint author are synonyms defined as:

coauthor and joint author: a writer who collaborates with others in writing something

coauthor (noun)	joint author (noun)
a writer who collaborates with others in writing something	a writer who collaborates with others in writing something