What is a 51% attack?
One miner or pool controlling most of the hashrate. They could reverse their own recent payments, but not steal coins or change the rules.
Whoever holds most of the hashrateHashrateHow many guesses a second all of Bitcoin's miners make together. More hashrate makes the network harder to attack. can, given time, build a longer chain than everyone else. With it they could undo their own recent payments, a double spendDouble spendAn attempt to spend the same bitcoin twice. Only one of the conflicting payments can ever confirm., or keep chosen payments out of blocks.
They cannot spend coins whose keys they don't hold, and they cannot change the rules: every node would still reject a block that broke them. Deeply confirmed payments stay impractical to reverse, which is why confirmations are the defence.
References
- Satoshi Nakamoto, “Bitcoin: A Peer-to-Peer Electronic Cash System”.