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

A 51% attack occurs when a single entity or group gains control of more than half (51%) of a cryptocurrency network's total mining hash rate or staking power. This allows the attacker to manipulate the blockchain, primarily by preventing new transactions from being confirmed or by reversing their own transactions, leading to double-spending.

It is more feasible on smaller, low-difficulty coins due to lower cost of acquiring the necessary hashing power.