A 51% attack refers to a scenario in blockchain networks where a single entity or group of entities controls more than 51% of the network's mining power, enabling them to manipulate the network's operations. With majority control, the attacker can potentially reverse transactions, double-spend coins, exclude or censor transactions, or rewrite the transaction history. A 51% attack undermines the decentralized nature and security of a blockchain network, highlighting the importance of a distributed network where no single entity can gain such control.
