The Bitcoin whitepaper is the original thesis paper written under the pseudonym Satoshi Nakamoto on October 31, 2008 that set the basic structure of the Bitcoin network. The paper is titled Bitcoin: A Peer-to-Peer Electronic Cash System. While one may imagine a thesis paper being exceptionally long, this one was only nine pages.
This was Satoshi's idea for an electronic currency that can make transactions with low costs while using no financial institutions or third parties. Instead of using a centralized server, it is structured to use many terminals connected over a peer-to-peer (P2P) network. It explains how the proof of work structure renders falsification of transaction information nearly impossible. For those who are interested, the original document can be read at https://bitcoin.org/bitcoin.pdf