Short, precise definitions for the terms used across this site. Hover any underlined word in the text to see the same entry inline.
A ledger that tracks mutable balances per address. Used by Ethereum, Solana, Algorand.
In the account model the global state is a mapping of address → balance. Transactions mutate sender and receiver balances directly. This makes contract programming feel familiar but introduces shared mutable state and contention.
The destination encoded into a UTXO. May be a public-key address or a script address.
A Cardano address combines a payment part (public key or script hash) and an optional staking part. UTXOs sitting at a script address can only be spent if the script (validator) permits.
An ordered batch of transactions appended to the chain by the slot leader.
A block contains transactions, a header referencing the previous block, and consensus metadata. On Cardano, slots elect a leader who has the right to produce the next block.
A new UTXO that returns the leftover value to the spender after a transaction.
Because a UTXO is consumed in full, any value beyond the target plus fee is returned to the spender as a new "change" UTXO. The change output is part of transaction construction, not optional.
The wallet’s job of picking which UTXOs to spend for a given target amount.
Because a UTXO must be spent in full, wallets need an algorithm to pick a set of UTXOs that covers a target amount. Strategies trade off transaction size, fee, change output, dust accumulation, and privacy. CIP-2 documents the strategies used in the Cardano ecosystem.
Custom data attached to an eUTXO. Acts as the “state” of a smart contract output.
A Datum is arbitrary data stored alongside a UTXO at a script address. When the UTXO is consumed, the validator sees the Datum and uses it together with the Redeemer and Script Context to decide whether to permit the spend. Think of it as a note pinned to a banknote that says what it is for.
A spend either succeeds for every observer or fails for every observer, with no global-state surprises.
Because eUTXO validators see only the transaction at hand, the outcome of submitting a transaction can be computed in advance. With honest pre-validation, transactions never fail on-chain unexpectedly. If a Plutus script does fail in phase-2 validation, only the dedicated collateral input is consumed — not the full transaction value. Compared to account-model chains where contracts can fail mid-execution and burn the full gas budget, this is a clear safety win.
A UTXO so small it costs more in fees to spend than it is worth.
Dust UTXOs accumulate when coin selection strategies leave behind tiny outputs. They fragment the wallet, hurt privacy, and may be uneconomic to ever spend on their own.
Extended UTXO. Cardano’s ledger model: a UTXO can carry datum and be locked by a script, enabling smart contracts on a UTXO chain.
Extended UTXO (eUTXO) is the model formalised by Chakravarty et al. (2020) and adopted by Cardano. It augments classical UTXOs with a Datum (state attached to the output) and a Validator script (a function that decides whether the output may be spent). Smart contracts gain expressivity while keeping the local-reasoning benefits of UTXO.
A pricing regime where users bid for inclusion. Higher fees mean faster confirmation.
A fee market emerges when blockspace is scarce and fees are user-chosen. Bitcoin and most UTXO chains operate this way. Wallets estimate the going rate, sophisticated users pay tips, and low-fee transactions can sit in the mempool for hours when the chain is busy. Cardano deliberately avoids a fee market: fees are computed as a·size + b with protocol-set constants, so two identical transactions always cost the same.
A node-local pool of pending transactions waiting to be included in a block.
Submitted transactions sit in the mempool until a block producer picks them up. On fee-market chains (Bitcoin, Litecoin, etc.) higher fees move a transaction up the queue. On Cardano fees are deterministic — fee = a·size + b — so there is no bidding and no fee market. On eUTXO chains, transactions only conflict if they spend the same UTXO, so independent transactions can be included in parallel without ordering constraints.
Data supplied by the spender of an eUTXO. The “action” passed into the validator.
A Redeemer is the input the spending transaction provides to the validator script: claim, swap, vote, cancel, whatever the contract expects. Together with the Datum and Script Context, it determines whether the validator returns true and the spend is allowed.
The full transaction visible to a validator: inputs, outputs, fees, signatories, time range.
The Script Context is the deterministic snapshot of the entire transaction that a validator can inspect. Because the validator sees the whole transaction (and only that), eUTXO contracts are predictable: you can know whether a spend will succeed before submitting it.
A one-second unit of time in Ouroboros consensus. Each slot may produce a block.
Cardano time is divided into one-second slots. In each slot a leader may be elected via Ouroboros Praos, with probability proportional to stake. Many slots remain empty by design; on average a block is produced every 20 seconds. Validity ranges in transactions are expressed in slots.
A bundle that consumes input UTXOs and creates output UTXOs, atomically.
A transaction in a UTXO ledger references existing unspent outputs as inputs and produces new outputs. It either applies in full or not at all. On Cardano it also carries fee, validity range, signatories, and any required redeemers and scripts.
Unspent Transaction Output. A discrete chunk of value sitting at an address, consumable in full by a future transaction.
A UTXO (Unspent Transaction Output) is the fundamental unit of value in the UTXO accounting model. Instead of holding a single mutable balance, a wallet holds a set of immutable outputs from prior transactions. Each spend consumes whole UTXOs and creates new ones.
A pure script that gates a UTXO. Returns pass or fail given Datum, Redeemer and Script Context.
A validator (sometimes called a script or smart contract on Cardano) is a function from (Datum, Redeemer, Context) to a boolean. The UTXO can be spent only if the validator returns true. Validators are stateless — any state lives in the Datum of the UTXO they protect.