51% attack

A 51% attack is a scenario in which a single entity or a coordinated group gains control over more than half of a Proof of Work network's computing power (or, correspondingly, more than half of the staked funds in a Proof of Stake network). Such an advantage theoretically enables blocking new transactions and even double-spending the same coins.

In practice, carrying out such an attack on large networks like Bitcoin or Ethereum is economically unviable — the cost of acquiring sufficient computing power or staked funds far exceeds the potential profit. The risk mainly applies to smaller, less decentralized blockchains with low total network power.