What is a 51% attack in cryptocurrency?

A 51% attack occurs when a group of miners or validators gains control of more than 50% of the computational power or staked tokens in a proof-of-work or proof-of-stake blockchain network. This majority control allows them to manipulate the blockchain by reversing transactions, double-spending coins, or halting new transactions from being confirmed. While a 51% attack does not allow attackers to steal coins directly, it undermines the integrity and security of the network. Cryptocurrencies like Bitcoin are designed to be resistant to such attacks due to their decentralized nature, but smaller networks with less hashing power are more vulnerable to these types of attacks.