With a 51 percent attack, a potential attacker manipulates a blockchain - or block - by combining the majority (>51%) of the computing power in a network.
Most cryptocurrencies operate according to the proof-of-work principle, a consensus mechanism that helps participants agree on an identical blockchain (or identical blocks) in a decentralized network. This is intended to ensure the security and transparency of blockchain applications at all times. However, while blockchain is undoubtedly one of the most fraud-proof technologies to this day, it is not perfect.
The so-called 51% attack (also majority attack) is a strategy to attack a blockchain network. Specifically, it is an attack vector in a proof-of-work-based network such as Bitcoin. In doing so, attackers bring up at least 51 percent of the computing power (hashrate, also known as hash power) in a network, i.e., the total available computing power of all miners/mining computer systems combined. By controlling the majority of the hashrate, an attacker can produce more blocks (or solve blockchain puzzles) in the long run than the rest of the network participants. They can leverage this to send a new version of the blockchain to the network in which individual transactions have been altered or removed. The attacker thus literally dictates - or censors - the history of the blockchain.
This allows the attacker to manipulate entries in the blockchain and, for example, reverse transactions or make so-called double spends: In a double spend, the same amount of a cryptocurrency (e.g. Bitcoin) is spent multiple times by the same person.