a writer who collaborates with others in writing something
"Sarah and her research partner decided to coauthor a new book about climate change after spending months gathering data together."
An author who collaborates with another to write something.
"My roommate and I decided to coauthor a guide on healthy eating because we both have expertise in nutrition."
To write something in collaboration with another author.
"The new history book will be coauthored by a university professor and a veteran journalist to ensure both academic rigor and engaging storytelling."