What is 51% Attack?

A 51% attack is a scenario in which more than 50% of a blockchain network's nodes fall under the control of a single group. In such a circumstance, the consensus of the network may no longer be sufficiently decentralized enough to be viable, leaving the blockchain open to manipulation. Attackers with greater than 50% control of a blockchain may be able to stop, reverse, and duplicate new transactions, a catastrophic condition for any blockchain. Notably, a 51% attack becomes more difficult and expensive as a network grows more sizable, decentralized, and valuable.