A majority attack where an individual or group controls more than half of the blockchain's mining power.
A 51% attack happens when a malicious actor gains control of a network's mining mechanism and is able to override the consensus algorithm that secures the blockchain.
The attacker may be an individual or a group of malicious miners working together. With a greater mining speed than everyone else, they are able to delay or stop other transactions from being confirmed.
A person or organization with more than 50% control of a blockchain's mining power is able establish a mining monopoly because they have the power to stall or disable other miners from mining.
The higher the proportion of the hash power held by the malicious agents, the greater the risks to the network. They can delay or alter transaction records, disrupt the network, reduce miner rewards, and reduce trust in the blockchain.