A 51% attack (or majority attack) refers to a potential attack on the integrity of a blockchain system in which a single malicious actor or organization manages to control more than half of the total hashing power of the network, potentially causing network disruption.
If a single bad user, or group of bad users acting together, control more than 50% of the total network hashing rate for a blockchain, they are able to override the consensus mechanism of the network and commit malicious acts such as double-spending. The attacker would have enough mining power to intentionally modify the ordering of transactions, preventing some or all transactions from being confirmed (aka. transaction denial of service). He would also be able to prevent some or all other miners from mining, leading to the so-called mining monopoly.