A freelancer is a person who works independently and is not permanently employed by a company. They offer their services or skills to different clients on a project basis, without being tied to a specific employer. Full definition
Web content writers work as freelancers for several clients. (copypeer.com)
However, this can be a real issue for freelancers as it can often be impossible to predict how long a project will take. (creativeboom.com)
It is not necessary to add the names of freelancers who worked on the book, especially if you're not listing them in the book itself. (authorsguild.org)