A 51% attack is when one person or group controls more than half of a blockchain, meaning they have a majority say when it comes to confirming transactions, for example.
Blockchains work because many validators agree on what is true. If someone controls over 51% of that power, they can cheat the system.
Having veto power over which transactions are accepted as truth means they can create false records that allow them to double-spend [→] or manipulate transaction history for personal gain.
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