A block is the basic unit of a blockchain — think of it as one page in the ledger. Each block packages a batch of transaction records, plus a timestamp and a fingerprint pointing to the previous block (the previous block's hash). Blocks link in order, head-to-tail by hash, forming the blockchain — which is where the name comes from.

The set of rules by which all nodes in a blockchain agree, without any central administrator, on what the next <a href="/en/glossary/blockchain-fundamentals/block/">Block</a> looks like and which transactions are valid. It's the core that lets a decentralized network operate securely and resist fraud; the two most common are <a href="/en/glossary/blockchain-fundamentals/proof-of-work/">Proof of Work</a> (PoW) and <a href="/en/glossary/blockchain-fundamentals/proof-of-stake/">Proof of Stake (PoS)</a>.

A mathematical function that turns input data of any length into a fixed-length, gibberish-looking digital fingerprint. Its key properties: the same input always gives the same output, a tiny change in input yields a wildly different output, and it's practically impossible to reverse the output back to the original input. It's the basic tool that links blocks and makes data tamper-evident on a blockchain.

Layer 2 (L2) is a second-layer protocol built on top of a main blockchain (Layer 1, like Ethereum), designed to dramatically improve transaction speed and reduce fees without sacrificing L1 security. L2's core logic: batch-process large numbers of transactions off-chain (in L2's own environment), then only submit the final results or proofs to Ethereum mainnet (L1) for verification and confirmation. Users experience fast, low-fee transactions on L2 while the underlying security is still guaranteed by Ethereum's consensus mechanism. Arbitrum, Optimism, Base, zkSync Era, and StarkNet are currently the most mainstream Ethereum L2s.

A <a href="/en/glossary/blockchain-fundamentals/merkle-tree/" target="_blank">Merkle Tree</a> is a tree-shaped data structure derived from cryptographic hash functions, developed by computer scientist Ralph Merkle in 1979. In blockchain, it's used to efficiently summarize and verify the integrity of all transactions in a <a href="/en/glossary/blockchain-fundamentals/block/">Block</a>: each transaction is hashed, pairs are hashed together, this process repeats level by level upward until a single Merkle Root is produced. This root is recorded in the <a href="/en/glossary/blockchain-fundamentals/block/">Block</a> header. Any tampered transaction changes its hash, cascading to change all parent hashes, ultimately changing the root — making tampering immediately detectable.

A <a href="/en/glossary/blockchain-fundamentals/node/" target="_blank">Node</a> is one of the many computers running blockchain software and participating in maintaining the chain. Each node stores a full (or partial) copy of the blockchain ledger and is responsible for verifying whether new transactions and new blocks are valid. Precisely because the ledger is kept and cross-checked by thousands of independent nodes worldwide, no single party can secretly alter the records — this is the physical basis of a blockchain's decentralization and tamper-resistance. The more nodes and the more dispersed they are, the harder the chain is to control or attack.

A blockchain <a href="/en/glossary/blockchain-fundamentals/consensus-mechanism/">Consensus Mechanism</a> where validators lock up tokens as collateral to earn the right to record blocks, rather than burning electricity to compete. Bad actors have their stake slashed — that internal penalty is the core mechanism keeping the ledger secure. Ethereum switched from PoW to PoS in 2022.

A <a href="/en/glossary/blockchain-fundamentals/consensus-mechanism/">Consensus Mechanism</a> where nodes compete by burning computing power to solve a guessing puzzle. Whoever finds a valid answer first gets to add the next <a href="/en/glossary/blockchain-fundamentals/block/">Block</a> and claim the reward — the system Bitcoin uses to keep its ledger tamper-resistant.

A <a href="/en/glossary/tokenization/smart-contract/" target="_blank">Smart Contract</a> is a piece of code deployed on a blockchain that automatically executes according to pre-written if-then logic — with no need for any intermediary (bank, lawyer, platform) to intervene or confirm. Once deployed on-chain, its rules are maintained by all nodes collectively, and no one can unilaterally modify or stop its execution. Everything you do in DeFi — swapping, lending, staking, providing liquidity — is smart contracts automatically executing business logic on your behalf.

In blockchains using <a href="/en/glossary/blockchain-fundamentals/proof-of-stake/">Proof of Stake (PoS)</a> consensus (like Ethereum and Solana), a validator is a special network participant: they lock a certain amount of cryptocurrency as staked collateral in exchange for the right to participate in <a href="/en/glossary/blockchain-fundamentals/block/">Block</a> proposals and transaction validation, earning corresponding <a href="/en/glossary/blockchain-fundamentals/block/">Block</a> rewards. Validators replace Bitcoin's PoW miners — no longer competing via hash power, but obtaining block production rights through staked capital amount and random selection mechanisms. If validators act maliciously or go offline too long, their staked collateral can be slashed.

A <a href="/en/glossary/blockchain-fundamentals/whitepaper/" target="_blank">Whitepaper</a> is the core technical and conceptual document for a cryptocurrency or blockchain project, typically published before the official launch or fundraising, explaining the problem it aims to solve, the technical design principles of its solution, the <a href="https://claude-me.com/en/glossary/core-concepts/token/">Token</a>'s function and distribution plan, and the team's roadmap. Bitcoin's whitepaper is the most important document in blockchain history — the nine-page paper Bitcoin: A Peer-to-Peer Electronic Cash System published by Satoshi Nakamoto in October 2008 laid the technical foundation for the entire cryptocurrency industry. Reading a project's whitepaper is one of the basic due diligence steps for evaluating whether it's worth investing in.

A cryptographic technique letting a prover convince a verifier that a statement is true without revealing any information about the statement itself. Example: prove you're over 18 without revealing your age; prove you have enough funds without revealing your balance. In blockchain, ZKP enables privacy and efficient <a href="/en/glossary/blockchain-fundamentals/smart-contract/">Smart Contract</a> verification. One of cryptography's most powerful tools.