Proof of work (PoW)
Originally advocated by Back in 2002, PoW is the consensus mechanism for the two leading blockchains: Bitcoin and Ethereum. Miners compete to find a hash, which is hard to find but easy to verify. Miners are rewarded for finding the cryptographic hash and using it to add a block to the blockchain. The computing difficulty of finding the hash makes it impractical to go backward to rewrite the history of a leading blockchain.