51% Attack

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A 51% attack refers to a situation when one person (or single group) controls over 50% of the total block creation, of a particular blockchain network, and they disrupt the integrity of the blockchain.

As consensus is a key requirement for the operation of blockchains, if a bad actor was able to acquire more than 50%, they would potentially be able to modify the recording process, preventing transactions from being confirmed or for double-spending (when crypto coins can be spent more than once).