Proof-of-work is a protocol used to electronically 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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