When one party controls enough hashpower or stake to rewrite recent blocks and double-spend.

An Application Binary Interface describes how software encodes contract calls and decodes results, events, and errors.

A ledger design that stores a balance and nonce per address, used by Ethereum and many smart-contract chains.

A destination on a chain where funds or tokens can be sent. Format varies by network (hex, bech32, base58).

Receiving twice to the same address, which links history. Bitcoin culture treats this as a privacy leak.

Key pairs where the public key verifies what only the private key can sign or decrypt.

A time-boxed review of contract code. It reduces risk. It does not eliminate it.

A batch of transactions plus a header that links to the previous block by hash.

The number of blocks from genesis to the current tip. A simple way to talk about chain progress.

Newly issued coins plus fees paid to the miner or validator that produced a valid block.

The target or observed interval between blocks on a network.

An append-only distributed ledger where blocks of transactions are cryptographically linked and validated by a network of nodes.

A public reward for reporting vulnerabilities before they are exploited.

The compiled form of a contract that the virtual machine actually runs.

A numerical identifier included in supported blockchain signatures to distinguish the network on which an authorization is intended to work.

When a node switches to a heavier or longer fork and drops recently accepted blocks.

The UTXO sent back to yourself when you spend more than the payment amount.

A fee-bumping trick that spends an unconfirmed output with a high-fee child so miners take both.

The first transaction in a Bitcoin block, which pays the block reward. Not related to the company Coinbase.

A wallet whose keys stay offline, often on a hardware device or paper backup, used for longer-term storage.

In common Bitcoin usage, inclusion in a block gives one confirmation, and each subsequent block on the accepted chain adds another.

Math that protects confidentiality, integrity, and authenticity. Blockchains rely on hashes and signatures.

Who holds the keys. A custodian can move funds on a client's behalf and takes operational and legal responsibility.

A decentralized application whose backend logic runs in smart contracts and whose users keep their own keys.

The BIP-44-style path (m/44'/60'/0'/0/0) that turns a seed into a specific coin address.

A more disposable developer network than testnet, often reset without notice.

A proof, created with a private key, that a specific message was authorized by the key holder.

A shared database replicated across many machines, of which a blockchain is one design.

Distributed ledger technology. The broader category that includes blockchains and other shared ledgers.

Binding a cryptographic message to a specific application, protocol, network, or purpose so that matching data is not confused across contexts.

Attempting to spend the same coins in two conflicting transactions. Consensus exists to make this fail.

The family of curves (secp256k1, Ed25519) used by most chains to make compact public keys and signatures.

Externally owned account. A key-pair wallet, the default Ethereum account type.

The Ethereum Virtual Machine, the runtime that executes smart contracts on Ethereum and many compatible chains.

A service that gives out testnet tokens so developers can pay fake gas.

A split in chain history or protocol rules. Can be accidental (two tips) or intentional (an upgrade).

Mathematically proving that code matches a spec, used on high-value vaults and zk circuits.

A node that verifies every block and transaction against consensus rules instead of trusting a third party.

The unit of computational work on Ethereum-style chains. Every opcode costs gas, paid in the network fee token.

The maximum gas a transaction or block may consume. A low limit can cause a transaction to fail mid-execution.

The amount paid per unit of gas, usually quoted in gwei; effective fees and inclusion depend on the network's fee rules and transaction conditions.

The launch parameters and first block of a chain or a token distribution.

The first block or agreed starting record of a blockchain, anchoring its initial history and state under that network's rules.

A rule change that old software will reject. Nodes must upgrade or they follow a different chain.

A dedicated device designed to keep signing keys isolated from a general-purpose computer and to authorize transactions through a controlled interface.

A fixed-length fingerprint of data produced by a one-way cryptographic function, used to detect tampering and link blocks.

The act of running data through a hash function so the output changes if even one input bit changes.

A wallet whose keys sit on an internet-connected device. Convenient for spending, weaker against remote theft.

The practical difficulty of changing confirmed history once enough honest work or stake sits on top of it.

The 256-bit Keccak hash function used in Ethereum; it is related to, but not interchangeable with, standardized SHA3-256.

An encrypted file that holds a private key, used by some desktop and node wallets.

A client that verifies selected authenticated blockchain data with less local computation and storage than a full node; its proofs and trust assumptions depend on the protocol.

The production chain where tokens have real value.

The waiting room of valid but unconfirmed transactions each node keeps before they land in a block.

The single hash at the top of a Merkle tree, stored in a block header as a compact commitment to every transaction.

A binary tree of hashes that lets a verifier prove a transaction is in a block without downloading the whole block.

A participant in proof of work that expends energy to find a valid block nonce and collect the reward.

An ordered word-based encoding used in wallet recovery; BIP-39 mnemonics encode entropy and a checksum before a separate step derives the wallet seed.

A wallet that needs M of N signatures before a spend clears. Used for treasuries and shared custody.

A computer that speaks a chain's peer protocol, checks rules, and optionally stores the full ledger.

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.

Creating many fake identities to outvote honest peers. Proof of work and stake make this expensive.

Making it expensive to fake many users, via stake, identity, or proof of personhood.

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.