A scenario where a single entity controls more than 50% of a proof-of-work blockchain’s hash rate (or proof-of-stake stake), allowing them to censor transactions, double-spend, or fork the chain. Bitcoin and Ethereum are too expensive to attack practically; smaller chains have suffered actual 51% attacks (Ethereum Classic, Bitcoin Gold). The cost of the attack vs. the value extractable is the key economic deterrent.
51% Attack
A scenario where a single entity controls more than 50% of a proof-of-work blockchain's hash rate (or proof-of-stake stake), allowing them to censor transactions, double-spend, or fork the chain. Bitcoin and Ethereum are too expensive to attack practically; smaller chains have suffered actual 51% attacks (Ethereum Classic, Bitcoin Gold). The cost of the attack vs. the value extractable is the key economic deterrent.
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