51% Attack

A 51% attack describes a situation where a single miner or group controls more than half of a proof-of-work network's computing power, or an equivalent majority stake in a proof-of-stake network.

With that level of control, an attacker could potentially reverse their own recent transactions or block others from confirming, though rewriting deep history or creating new coins outside the protocol's rules generally remains impractical. See Kaavon's "Proof of Work" and "Proof of Stake" glossary definitions for the two consensus mechanisms this specific risk applies to.