Whitepaper
Document describing a crypto project technically and economically.
A project's technical plan: what problem it solves, how it works technically, how tokenomics is structured, who's behind it. Bitcoin's whitepaper (Satoshi, 2008) is 9 pages, the gold standard. Most 2017-21 ICO whitepapers were copy-pasted nonsense with no code. Today professional projects also publish technical docs, roadmaps and code audits.
In plain words
A document where the project explains: what problem it solves, how it works technically, what the tokenomics is, who's behind it. The Bitcoin whitepaper is 9 pages and remains the gold standard. If you don't understand a whitepaper, or it reads as marketing with no substance, it is a red flag.
Example
Satoshi's Bitcoin whitepaper (October 31, 2008). 9 pages, 12 sections, still the gold standard.