51% Attack is defined as a situation where more than half of the computer power running a blockchain are controlled by one person or one group of people with evil intentions. That person or group with the majority power can manipulate transactions in their favor.
In a 51% attack, the attackers might be able to:
- Prevent new transactions from being included in the blockchain.
- Reverse transactions so they could spend the same money again and again, known as a double-spend.
There are theories that it may even take less than the majority computing power (51%) to manipulate a cryptocurrency.