A 51% attack occurs when someone takes control of more than half of the computing power (in the case of a proof of work blockchain) or strike power (in the case of a proof of stake blockchain). This majority allows the attacker to take control of the network. This allows them to undo transactions, duplicate spending, or even influence the issuance of new tokens. It is a serious threat in the crypto world because it can undermine the integrity and reliability of the entire blockchain.