A white paper is a form of persuasive writing that advocates a certain product, service, or belief. There is no defined standard for white papers. Anyone can call anything a white paper. Generally, however, they function as a stand-alone document, propose a solution to a problem, and are targeted at a larger audience outside of the organization that authored the paper. White papers are often written to convince potential customers to buy a product or service, although better white papers will be genuinely educational and useful to decision makers and not simply a commercial in essay form.
You may want to review the cyber-security white papers at the National Institute of Standards for examples of how white papers can be written. See csrc.nist.gov/publications/white-paper.