Proof of work
Proof of work is a protocol used electronically to verify transactions made on a distributed ledger network. It is the set of rules for the process by which many cryptocurrencies (like bitcoin) are mined.
Proof of work is described in Satoshi Nakamoto's bitcoin whitepaper, Bitcoin: A Peer-to-Peer Electronic Cash System.  Along with transaction blocks, proof of work is one of the outputs of mining. In bitcoin, the proof of work requires building a block of transactions and solving the hash function.
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