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What is an Unspent Transaction Output (UTXO)?

02 Feb 2023
4 Minute Read

The meaning of Unspent Transaction Output (UTXO) is a term used in the cryptocurrency space to describe an unspent transaction output that can be used as an input to another transaction. UTXOs form the basis of the Bitcoin network, allowing users to send and receive funds denominated in bitcoins or other tokens. In essence, UTXO describes the amount of money available for transfer from one user’s wallet, without any value being taken out of it. This makes it extremely efficient for transactions on blockchain networks such as Bitcoin where every single transaction requires its own dedicated UTXO.

When a user sends a transaction on a blockchain network, their wallet will create one or more outputs that are sent along with the payment. The amount of money sent is referred to as the “input,” while the amount of money left over after all fees and other expenses are paid out is referred to as the “output” or UTXO. Once these outputs become part of a block, they become an unspent transaction output for that user. All subsequent transactions must include at least one UTXO from that user in order for them to be complete.

It’s important to note that UTXOs don’t necessarily have to have one-to-one correspondence with wallet addresses; rather, different wallets can share a common UTXO pool between them. This allows users to move their funds between wallets without having to create a new UTXO for each transfer.

Overall, Unspent Transaction Outputs are an important part of the cryptocurrency ecosystem as they enable users to quickly and securely send money without having to create a new UTXO each time a transaction is sent. As such, many exchanges and wallets make use of UTXOs in order to facilitate transactions between users and other services on their respective networks. For this reason, it’s important that users understand how UTXOs work in order to ensure secure and efficient transfers when conducting any type of digital asset transaction.

Simplified Example

Unspent Transaction Output (UTXO) is like coins in a piggy bank. Imagine you have a piggy bank where you put all your coins, and every time you want to spend some money, you take out certain coins from the piggy bank. UTXO works in a similar way, each time a transaction occurs, it creates new outputs which are like new coins added to the piggy bank and the remaining part of the coins are still in the piggy bank, they are unspent and can be used in future transactions. Each of these unspent outputs is called UTXO.

Who Invented the Unspent Transaction Output?

The conceptual foundation of the UTXO (Unspent Transaction Output) model can be attributed to Hal Finney's Reusable Proofs of Work proposal, which, in turn, drew inspiration from Adam Back's 1997 Hashcash proposal. The UTXO model found its practical implementation with the release of Bitcoin in 2009, marking the pioneering adoption of this model on a broad scale.

Examples

Bitcoin Transactions: In the context of the Bitcoin blockchain, an unspent transaction output (UTXO) refers to a previously unspent amount of bitcoins that has been received as part of a transaction but has not yet been spent in a subsequent transaction. UTXOs are the building blocks of transactions on the Bitcoin network and are used to represent the available spendable balance of a Bitcoin address.

Cryptocurrency Wallets: UTXOs are used by cryptocurrency wallets to keep track of the available spendable balance of a user's cryptocurrency holdings. A wallet may contain multiple UTXOs, each representing a different transaction input, and the wallet will spend these UTXOs as part of new transactions to pay for goods and services or to transfer cryptocurrency to other addresses.

Blockchain Accounting: UTXOs play a critical role in the accounting of cryptocurrencies on the blockchain, as they represent the current state of the network's unspent balances. By tracking the UTXOs on the blockchain, nodes can validate and verify transactions, ensuring that the cryptocurrency balance of a user's address is accurate and up-to-date. This helps to prevent double spending and other types of fraud on the blockchain.

  • Wallet: A secure digital wallet used to store, send, and receive digital currencies such as Bitcoin, Ethereum, and Litecoin.

  • Digital Asset: A type of financial asset that exists only in digital form and is stored and traded on electronic networks.

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