What Is a 51 Percent Attack in the Context of Cryptocurrency Mining?

A 51 percent attack occurs when a single entity or group controls more than half (51 percent) of a cryptocurrency network's total mining hashrate. This control allows them to effectively monopolize the process of adding new blocks and validating transactions.

With majority control, the attacker can prevent other miners from completing blocks, reverse their own transactions (double-spending), and stop or censor specific transactions. This fundamentally undermines the security and immutability of the blockchain.