A 51% attack is a specific type of blockchain attack that occurs when a single entity gains majority control (over 50%) of a network’s mining hash rate or staking power, enabling them to manipulate transactions. While a 51% attack can facilitate double-spending, where the attacker reverses transactions to spend the same coins twice, it differs from standalone double-spending attacks in scope and method. Traditional double-spending can occur in weaker consensus mechanisms or poorly secured networks without requiring majority control, such as through race attacks or Finney attacks, where malicious actors exploit timing delays in transaction confirmations. In contrast, a 51% attack grants broader control, allowing not just double-spending but also transaction censorship, blockchain reorganisation, and even temporary halting of new blocks. Additionally, while double-spending typically targets individual transactions, a 51% attack threatens the entire network’s integrity, undermining trust in the blockchain’s immutability. However, both attacks exploit vulnerabilities in consensus mechanisms, with double-spending being a potential consequence of a 51% attack rather than a separate attack vector. Larger, more decentralised networks like Bitcoin are highly resistant to 51% attacks due to their immense hash power, whereas smaller blockchains with lower participation are more susceptible to both 51% attacks and isolated double-spending incidents.