Consensus

Consensus is how a decentralized network reaches a single agreed-upon version of the ledger. Thousands of nodes operated by strangers must all accept the same next block, even though some may be offline, slow, or actively trying to cheat. A consensus mechanism is the set of rules that makes this possible.

Proof of work, used by Bitcoin, selects the next block through a computational race and treats the chain with the most accumulated work as the true one. Proof of stake, used by Ethereum, Solana, and most newer chains, selects block proposers from validators who have locked up coins, and penalizes them if they sign conflicting blocks. Other designs exist, but these two dominate.

A good consensus mechanism makes attacks expensive. To rewrite Bitcoin's history an attacker would need more hashing power than the rest of the network combined. To attack Ethereum, they would need to control a large share of all staked ETH and would lose it in the process. The security of a chain is ultimately the cost of defeating its consensus.