A white paper is a document that provides information about a service, technology or product. Most white papers are written in an academic style & focus on providing technical information to prospective investors & users.
The Bitcoin white paper is a nine-page document titled: "Bitcoin: A Peer-to-Peer Electronic Cash System." It was published on the 31st of October, 2008, by an individual or a group under the pseudonym of Satoshi Nakamoto.
The Bitcoin white paper describes an electronic currency that can make transactions with low costs, while using no financial institutions or third parties. Instead of using a centralised server, it is structured to use many terminals connected over a global peer-to-peer network. It explains how Bitcoin transactions work, how network consensus & verification process is achieved, how the network is secured against attacks, & how the proof-of-work structure renders falsification of transaction information nearly impossible.
The white paper marks the birth of Bitcoin as we know it today.