What is 51% Attack?
A 51% attack happens when a single entity controls more than half of a Proof of Work network's hash rate, allowing them to censor transactions, exclude blocks, or reorganise recent history to double-spend. The attack does not let them steal coins from arbitrary wallets or break cryptography.
Why 51% Attack matters
Understanding 51% Attack is part of building a solid mental model of how Bitcoin, blockchain and Web3 systems actually work. Concepts in the Security category sit at the foundation of the broader stack — get them right and the rest is far easier.
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Related terms
- Proof of Work — A consensus mechanism that requires computational work to add blocks.
- Hash Rate — The total computational power securing the network.
- Double Spend — Attempting to spend the same coin twice.
- Reorganization (Reorg) — Replacing recent blocks with a competing chain that has more work.
More security terms
- Seed Phrase — A human-readable backup of a wallet's private keys.
- Wallet — Software or hardware that manages your private keys.
- Cold Storage — Keeping private keys offline for security.
- Custody — Who controls the private keys.
- Slashing — Destroying part of a validator's stake as punishment.
- Multi-Signature (Multisig) — A wallet requiring multiple keys to spend.
- Hardware Wallet — A dedicated device that signs transactions offline.
- Hot Wallet — A wallet whose keys live on an internet-connected device.
Keep exploring
Continue with the full blockchain glossary — 136 terms in total — or read the developer blog and FAQ for deeper context.