What Is a ‘51% Attack’ in the Context of Blockchain Consensus Mechanisms?

A 51% attack occurs when a single entity or coordinated group gains control of more than half (51%) of the network's total computational power (in PoW) or staked currency (in PoS). With this majority control, the attacker can manipulate the blockchain, such as preventing new transactions from being confirmed, stopping other miners/validators from mining, and, most critically, reversing their own transactions, leading to 'double-spending'.