joint author vs author

joint author and author both are nouns.

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

As nouns, author is a hypernym of joint author; that is, author is a word with a broader meaning than joint author:

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

Other hypernyms of joint author include writer.

joint author (noun)	author (noun)
a writer who collaborates with others in writing something	writes (books or stories or articles or the like) professionally (for pay)
	someone who originates or causes or initiates something

joint author (verb)	author (verb)
	be the author of