Consensus
Mechanism by which nodes agree on the validity of new blocks.
Consensus solves trust without central authority. Types: PoW (Bitcoin: mining, energy-heavy, most secure), PoS (Ethereum, Solana: validators stake tokens, eco-friendly), DPoS, PoA, PoH.
In plain words
The mechanism by which thousands of computers worldwide agree on which version of the blockchain is correct. Without it, anyone could write 'I have 1000 BTC' for themselves. Bitcoin uses Proof of Work (first to solve the puzzle wins), Ethereum uses Proof of Stake (most tokens locked wins). All crypto security starts here.
Example
BTC: PoW, 51% attack costs ~$30B hardware. ETH: PoS, validator needs 32 ETH staked, 51% attack ~$50B + slashing risk.
Also known as: consensus mechanism, konsensus blockchain