What is a 51% attack in blockchain?

A 51% attack occurs when an individual or group gains control of more than 50% of the computational power (in proof-of-work systems) or the staked tokens (in proof-of-stake systems) on a blockchain network. With majority control, they can manipulate the network by reversing transactions, double-spending coins, or halting new transaction confirmations. This type of attack can undermine the integrity of a blockchain and erode trust in the network. While 51% attacks are difficult to execute on large, decentralized networks like Bitcoin, smaller blockchains are more vulnerable to this kind of attack due to their lower hashing power or staked assets.