A number used once, typically adjusted by miners until a block hash meets the network difficulty target.

Credentials, names, or soulbound tokens used as a durable persona across apps.

A Bitcoin opcode used to attach a small data payload, the ancestor of many inscription schemes.

A network where nodes talk to each other directly instead of through a single server.

A DLT where only approved nodes validate. Common in bank and enterprise pilots, unlike public permissionless chains.

A secret number that authorizes spends. If it leaks, the address is compromised.

A scheme to show a wallet is one human, used to fight airdrop farming and bots.

The key that can replace a contract's implementation. A centralized upgrade path.

The counterpart to a private key. Addresses are usually a hashed or encoded form of the public key.

A bug where a contract is called again before it finishes updating state. The DAO hack's family of bugs.

A Bitcoin policy that lets you bump a stuck transaction by republishing with a higher fee.

Rebroadcasting a valid signed transaction on another chain or after a fork so it executes twice.

The locking script on a Bitcoin output that defines how it can be spent.

A word-based wallet recovery backup, often using BIP-39; successful restoration can also require a passphrase, derivation paths, and the correct wallet format.

Control of the keys or authorization policy needed to use an on-chain account, with responsibility for protecting and recovering that authority.

A 256-bit hash function from the SHA-2 family. Bitcoin uses it for proof of work and address hashing.

Code deployed on a blockchain that executes deterministically when called.

A contract account with social recovery, spending limits, or sponsored gas, rather than a raw EOA.

A tighter rule set that old nodes still see as valid, so the network can upgrade without a clean split.

A parallel chain with valueless tokens used to try software before mainnet.

A signed instruction to move value or call a contract. Identified by a transaction hash or txid.

An account's transaction sequence number, used on Ethereum to order outgoing transactions and prevent the same signed transaction from executing repeatedly.

The hash that uniquely identifies a transaction on a given chain.

An unspent transaction output. Bitcoin-style ledgers track coins as discrete outputs rather than account balances.

A participant in proof of stake that proposes or attests to blocks and can be slashed for cheating.

Software or hardware that holds keys and builds transactions. It does not store coins. The chain does.

The signature data in a transaction. SegWit moved it to a separate field.

A token on one chain that represents an asset locked elsewhere, introducing custody and bridge risk.