Can you describe what a white paper is and what its purpose is?

While it is very likely that the interviewer knows what a white paper is, they are asking you this question to understand how you define it and see if your definition is the same as theirs. This allows you to demonstrate your expertise with white papers and describe what the company can expect if they hire you. You may even want to offer to provide them with samples of your work. Of course, this assumes that you've written white papers in your previous positions. If not, describe what a white paper is and confirm your ability to create one.