What Is a 51% Attack and What Is Its Potential Impact on a Cryptocurrency Network?

A 51% attack occurs when a single entity or group gains control of more than half (51%) of the network's total mining hash rate. With this majority power, the attacker can effectively control the ordering of transactions and prevent new transactions from gaining confirmation.

Critically, they can perform "double-spending," where they reverse their own transactions, severely undermining the integrity and trust in the blockchain.