51% Attack
An attack where one party controls most of a network's mining power and can rewrite recent transactions.
Proof-of-work chains assume no one controls more than half the hash power. Cross that line and the majority miner can reorder or reverse recent blocks, enabling a double spend — pay someone, then erase the payment from the chain's history.
It can't steal coins from existing wallets or change old, deeply buried transactions. The damage is limited to recent activity. Bitcoin is too expensive to attack this way, but smaller proof-of-work coins like Ethereum Classic and Bitcoin Gold have been hit, with exchanges losing millions.
Related terms
Spending the same crypto twice by getting two conflicting transactions accepted, the core problem blockchains were built to stop.
Proof of Work (PoW)A consensus method where miners burn computing power to earn the right to add blocks and deter cheating.
Hash RateThe total computing power a proof-of-work network throws at mining, measured in hashes per second.
MiningThe process of competing with computing power to add new blocks and mint fresh coins under proof of work.