In plain English
What 51% attack means
A 51% attack happens when an attacker controls more than half of the network’s relevant consensus power. On proof-of-work chains, that is mining hash rate; on some proof-of-stake systems, it is majority stake. With that power, the attacker may be able to reorganize recent blocks, censor transactions, or spend the same coins twice.
Why it matters
This is a core network-security risk for smaller or less distributed blockchains. It does not usually let an attacker steal arbitrary funds from wallets, but it can undermine transaction finality, confidence in settlement, and exchange deposit safety. The practical impact depends on the consensus design and how much power the attacker controls.
Example
If an attacker can mine faster than the honest network for a short period, they might send coins to an exchange, withdraw assets after the deposit is credited, and then reorganize the chain to remove the original payment.
Quick answers
Common questions
Does 51% mean full control of a blockchain?+
No. It means majority consensus power, which can enable certain disruptions, but it does not automatically give the attacker every possible network ability.
Can proof-of-stake networks face 51% attacks?+
Yes. The mechanism differs, but majority control of stake can still create a majority attack risk.
Sources