This definition comes from the technical glossary.
An 80-byte header belonging to a single block which is hashed repeatedly to create proof of work.
Block header — Bitcoin.org Developer Reference
Block hashing algorithm — Bitcoin Wiki
What are Bitcoin miners really solving? — Bitcoin StackExchange
Bitcoin.org is community supported: 1GwV7fPX97hmavc6iNrUZUogmjpLPrPFoE