A 51% Attack is a form of attack on a decentralized blockchain. The strength of a decentralized blockchain lies in the number of nodes maintaining the network. To make any change to the blockchain the nodes of the blockchain must come to a consensus. These changes usually happen in the form of mining, or adding new transactions to the ledger. When over 51% of the nodes agree on a change, the blockchain is updated with the new information.
A 51% Attack happens when a single entity, or a group working together, attempts to take control of at least 51% of a blockchain’s nodes or mining hash rate. If they succeed, the attackers gain control of the blockchain. They can choose which transactions get approved, stop transactions altogether, or reverse transactions to allow for double-spending. The more robust the network of the blockchain, the more difficult a 51% attack becomes. For example, Bitcoin’s network is robust enough to make a 51% Attack is still hypothetical, but basically impossible.