Definition
A 51% attack (or majority attack) occurs when a single entity or coordinated group controls more than half of the Bitcoin network’s total hashrate. With majority hashrate, the attacker could mine a private chain faster than the honest network and eventually release it to overwrite recent transaction history.
This would allow double-spending (spending the same Bitcoin twice) or transaction censorship. However, the enormous cost of acquiring 51% of Bitcoin’s hashrate (hundreds of billions in hardware and electricity) makes this attack economically impractical. This is why Bitcoin’s high and well-distributed hashrate is essential for security.
In Simple Terms
A theoretical attack requiring over 50% of network hashrate to double-spend or censor transactions.
A theoretical attack where an entity controlling more than 50% of the network hashrate could double-spend transactions or censor blocks.