If more than half the computer power or mining hash rate on a network is run by a single person or a single group of people,, then a 51% attack on an operation.

A 51% attack occurs when a single entity or group gains control of more than 50% of a blockchain network’s mining or staking power, allowing them to manipulate transactions. With majority control, attackers can:

Double-spend coins (reverse confirmed transactions).

Prevent new transactions from being confirmed.

Exclude other miners from validating blocks.

This attack threatens blockchain security and trust, particularly on smaller networks with lower hash rates (e.g., Bitcoin Gold, Ethereum Classic). Proof-of-Work (PoW) blockchains are more vulnerable than Proof-of-Stake (PoS) systems, though PoS can also face similar risks if a validator gains excessive stake.

Preventative measures include increasing decentralisation, implementing checkpointing, or using hybrid consensus mechanisms. A successful 51% attack can severely damage a cryptocurrency’s reputation and value.