Blockchain glossary
Definitions for the terms used across the lessons. Entries are linked from lesson prose, so you can jump from a concept to its reference definition.
26 terms across 5 categories
Blockchain
- block
- A batch of transactions plus metadata such as a previous block hash, timestamp, and consensus-specific proof.
- blockchain
- An append-only ledger made from blocks that reference earlier blocks by hash, so tampering with old data becomes detectable.
- fork
- A temporary or permanent split in chain history, usually caused by simultaneous blocks, rule changes, or attacks.
- mempool
- A node's local pool of valid, unconfirmed transactions waiting to be included in a block.
Cryptography
- digital signature
- A cryptographic proof that the private-key holder authorized a specific message, without revealing the private key.
- hash
- A fixed-size fingerprint of data. Good cryptographic hashes are deterministic, hard to reverse, and change unpredictably when the input changes.
- hash function
- A one-way function that maps arbitrary data to a fixed-length digest, used for block IDs, Merkle roots, mining, and integrity checks.
- merkle tree
- A tree of hashes that lets a node prove one transaction is included in a large block with only a logarithmic-size proof.
- nonce
- A number changed repeatedly during mining or signing. In signatures, nonce reuse can expose the private key.
- private key
- The secret number that controls an address. Whoever can sign with it can authorize spending from that address.
- public key
- A value derived from a private key that lets others verify signatures without learning the private key.
- seed phrase
- A human-readable backup that deterministically recreates wallet private keys. Losing it can make funds unrecoverable.
- zero-knowledge proof
- A proof that a statement is true without revealing the private witness or underlying secret inputs.
Consensus
- 51% attack
- A majority-resource attack that can rewrite recent history or censor transactions, but cannot steal funds without private keys.
- consensus
- The process by which independent nodes converge on the same valid chain history despite latency, faults, and attackers.
- finality
- The point at which reverting a transaction becomes economically, probabilistically, or protocol-level impractical.
- mining
- The Proof-of-Work process of searching for a nonce that makes a candidate block hash satisfy the difficulty target.
- proof of stake
- A consensus mechanism where validators lock capital and can be penalized for provable misbehavior.
- proof of work
- A consensus mechanism where miners spend real computation to find a block hash below the network target.
- slashing
- A Proof-of-Stake penalty that destroys part of a validator's stake for provable misbehavior such as double-signing.
- sybil attack
- An attack where one actor creates many fake identities. Blockchains resist it by tying influence to scarce resources such as work or stake.
- validator
- A Proof-of-Stake participant that proposes or attests to blocks and risks penalties for violating consensus rules.
Ethereum
- gas fee
- The fee paid for computation and storage on Ethereum-like networks, priced so scarce block space is not free to consume.
- smart contract
- Program code deployed on a blockchain that executes deterministically according to network rules.
Security
- double spend
- An attempt to spend the same funds twice by exploiting transaction ordering or rewriting recent chain history.
- reentrancy attack
- A smart-contract bug where external code calls back into a contract before its internal state has been safely updated.