An attack in which malicious actors gain control over 51 percent of nodes in a network. In The term "Bitcoin" can either refer to Bitcoin the network, ... More, Proof-of-Work (PoW) has been the main method of securing against double-spend attacks. A 51% attack on PoW means that malicious actors controlling more than 50% of the network’s mining hash can use this power to prevent new transactions from being confirmed or reverse transactions that were completed under their control, leading to a double-spend attack. Satoshi Nakamoto assumed in the Bitcoin whitepaper that acquiring 51% of Bitcoin’s hashrate would be impossible and thus did not consider the economic incentives behind a 51% attack. However, the biggest cost resulting from a 51% attack is the loss of confidence in the A blockchain is a shared digital ledger, or a continually up... More. Blockchains such as Bitcoin and Ethereum are believed to be secure from 51% of attacks. On the other hand, smaller blockchains/altcoins may be prone to this kind of attack.