Glossary
Linkable definitions of the terms used across the tracks. Each entry has its own anchor.
- Hash
- A fixed-size digest produced from arbitrary input by a one-way function. Cryptographic hashes are preimage- and collision-resistant, which is what lets a blockchain link blocks tamper-evidently.
- Nonce
- A number used once. In proof of work it is the value miners vary to find a valid block hash; in accounts it is a per-sender sequence number that prevents transaction replay.
- Merkle root
- The single hash at the top of a Merkle tree. It commits to an entire ordered set of items, so a block header can summarize thousands of transactions in 32 bytes and support O(log n) inclusion proofs.
- Finality
- The guarantee that a committed transaction can no longer be reversed. It is probabilistic on proof-of-work chains (stronger with each confirmation) or explicit/economic on many proof-of-stake chains.
- Validator
- A participant that proposes and/or attests to blocks. In proof of stake a validator’s influence is proportional to its staked capital, which can be slashed for misbehavior.
- Slashing
- The destruction of part of a validator’s stake as a penalty for a provable offense, most importantly equivocation (signing two conflicting blocks for the same slot). It makes attacks economically self-defeating.
- Mempool
- The in-memory pool of validated, pending transactions a node holds before they are included in a block. Producers select the highest-fee transactions from it.
- Gas
- A unit measuring the computational effort of an operation. Each instruction costs gas, a transaction carries a gas budget, and execution halts when the budget is exhausted — bounding untrusted code.
- Rollup
- A layer-2 design that executes transactions off-chain and posts compressed data (and, for ZK rollups, a validity proof) back to the base layer, inheriting its security while raising throughput.
- Sequencer
- The component of a rollup that orders transactions and produces batches. Most production rollups today run a single, centralized sequencer, an active area of decentralization work.
- Light client
- A client that verifies parts of a chain without storing all of it, typically by checking block headers and Merkle inclusion proofs rather than re-executing every transaction.
- Fork choice
- The deterministic rule a node uses to pick the canonical chain when it sees competing valid blocks — for example, most accumulated work, or the greatest weight of validator attestations.
- BFT
- Byzantine Fault Tolerance: the ability of a consensus protocol to reach agreement despite some participants being arbitrarily malicious or faulty, typically tolerating up to one-third of voting power.
- Liveness
- The property that a system keeps making progress — new transactions eventually get included and finalized. Contrast with safety.
- Safety
- The property that the system never enters an inconsistent state — for example, two conflicting blocks are never both finalized. Often traded against liveness under network partitions (see CAP).
- Sybil resistance
- A mechanism that makes it costly to gain disproportionate influence by creating many fake identities, by tying influence to a scarce resource such as computing power (PoW) or staked capital (PoS).
- Genesis block
- The first block of a chain (height 0), with no parent. All nodes must agree on it byte-for-byte; every later block links back to it through the chain of parent hashes.
- Consensus mechanism
- The combination of a Sybil-resistance method and a fork-choice rule that lets a permissionless network of mutually distrusting nodes agree on one ordered history.