What is a white paper?

It’s a formal document by the government, a person or company that presents an argument for something, be it a product or some other solution. 

Take for example the Bitcoin white paper, written in 2008 by Bitcoin creator Satoshi Nakamoto:  

“Abstract. A purely peer-to-peer version of electronic cash would allow online payments to be sent directly from one party to another without going through a financial institution. Digital signatures provide part of the solution, but the main benefits are lost if a trusted third party is still required to prevent double-spending. We propose a solution to the double-spending problem using a peer-to-peer network.”
The Bitcoin white paper continues to explain in technical detail how transactions will happen on the Bitcoin network, and the role of proof-of-work in the mining process, among other detailed breakdowns of this exceptional technological breakthrough.