White paper is more widely used and it has strong documentation to back it up. At the end of the day, white paper is the safer choice. You won't be wrong for choosing to use the space-separated version. When it comes down to it, whitepaper is an acceptable preference at best, while white paper is the standard.
(from blog linked above, May 12, 2016)
A white paper is a research-based report which offers a focused description of a complex topic and presents the point of view of the author or body represented by the author. The purpose of a white paper is to give readers understanding of an issue, which in turn helps them solve a problem or make a decision.
The term originated in Britain, where it refers to a type of government issued document. In a business context, the purpose of white papers has evolved to an aspect of marketing and is often used to persuade.
White paper experts including Gordon Graham have identified these key characteristics for a white paper:
If a document has all these characteristics, it’s probably a white paper
"The purpose of a white paper is to promote a certain product, service, technology or methodology, and to influence current and prospective customers' decisions. Many white papers are designed for B2B (business to business) marketing purposes, such as between a manufacturer and a wholesaler, or between a wholesaler and a retailer. The white paper is used to inform and persuade the other company that a certain offering (such as a product or technology) is superior for solving a particular business problem or addressing a certain challenge."
from Investopedia
Read more: White Paper https://www.investopedia.com/terms/w/whitepaper.asp
Countervailing View
"A whitepaper is a persuasive, authoritative, in-depth report on a specific topic that presents a problem and provides a solution.
Marketers create whitepapers to educate their audience about a particular issue or explain and promote a particular methodology. They're advanced problem-solving guides. Typically, whitepapers require at least an email address for download (usually they require information more than that), making them great for capturing leads."
A whitepaper is NOT:
"A product pitch.
Although Investopedia, [see above],defines a whitepaper as 'an informational document issued by a company to promote or highlight the features of a solution, product, or service', be warned that overtly shilling your own stuff could turn off your readers. The goal of a whitepaper is to inform and persuade based on facts and evidence, not tell the world why your product is the best and they need to buy it now."
