What is a 51% attack?

A 51% attack refers to a situation in which an individual or group gains control of more than 50% of a blockchain network's mining power or hashing power. In such a scenario, the controlling entity can potentially disrupt the network's operations. They could manipulate transaction confirmations, double-spend coins, or prevent certain transactions from being processed, thus undermining the trust and reliability of the blockchain system.

This attack is particularly concerning in proof-of-work cryptocurrencies like Bitcoin, where the security and integrity of the network rely significantly on the decentralized nature of mining power. By controlling the majority of this power, an attacker can exert undue influence over the network, leading to significant vulnerabilities and risks for users and merchants who rely on the blockchain for secure and trustworthy transactions.

In contrast, the other options do not accurately represent the concept of a 51% attack. The methods to increase transaction speed, types of cryptocurrency wallets, or security protocols for validating transactions are unrelated to the implications of controlling a majority of the mining power within a blockchain network.