What Is a 51% Attack and How Does It Relate to Decentralized Consensus?

A 51% attack occurs when a single entity or group gains control of more than half (51%) of a cryptocurrency's network computing power, or hashrate, in a Proof-of-Work system. This level of control allows the attacker to manipulate the transaction order, stop new transactions from being confirmed, and most critically, perform "double-spending." It directly undermines the security and integrity of the decentralized consensus mechanism, as the attacker can force their version of the blockchain to be accepted as the canonical chain.