What Is a 51% Attack in the Context of a Proof-of-Work Cryptocurrency?

A 51% attack occurs when a single entity or group controls more than half (51%) of the network's total mining hash rate. This control allows them to exclusively validate new blocks and, crucially, to reverse recent transactions.

The primary goal is usually double-spending, where an attacker spends coins and then uses their majority power to rewrite the blockchain, nullifying the initial transaction. This undermines the network's security and trust.

It is easier on coins with low total hash rate.