Technical Definition
51% attack
A 51% attack occurs when an attacker gains majority influence over a blockchain's relevant consensus resource and can interfere with transaction ordering or chain selection.
By Crypto University Editorial
ConsensusProof-of-workProof-of-stake
✦ Key Insight
Majority control can enable censorship, reorganization attempts or double-spending attacks under certain conditions.
✕ Common Misconceptions
"51%" is a convenient label, not a universal threshold for every attack on every blockchain.
Detailed Explanation
In proof-of-work this generally refers to controlling most mining power. In proof-of-stake, the relevant resource is stake, although exact attack thresholds and capabilities depend on the protocol.
In Practice
“Ethereum documentation explains that 51% of staked ETH can provide significant fork-choice and censorship influence, while rewriting finalized history requires stronger conditions.”