Glossary
51% Attack
An attack where a single party controls a majority of the network's hashrate or stake, letting them reorder, censor, or reverse recent transactions.
A 51% attacker can outpace the honest network at producing blocks, which lets them mine a private chain longer than the public one and then publish it — overwriting recent history. The usual goal is a double-spend: pay someone in the public chain, then make the transaction disappear in the rewritten one.
The attacker cannot steal coins they don't already own, mint new coins, or alter deep history. Small PoW chains (Ethereum Classic, Bitcoin Gold) have been 51%-attacked; doing it to Bitcoin or Ethereum mainnet would cost billions.