The consensus mechanism utilized by Bitcoin, first implemented by Adam Back and later popularized by Satoshi in The Bitcoin White Paper. It was also used by Ethereum until The Merge in 2022 migrated Ethereum to proof-of-stake.
Proof-of-work is a lottery whereby miners expense energy through hashing in an attempt to generate a hash with a specific number of leading zeros - known as the difficulty. The only way to generate a hash which can be verified by other nodes on the network is the do the work - spend the energy - which can be seen as proof that the hash was generated by following the rules of the network.
Word missing from the glossary or definition needs improvement? Let us know!