Unspent Transaction Output

The Unspent Transaction Output model is a fundamental design pattern for managing account balances in many blockchains, including Bitcoin. In this system, there is no explicit account balance stored; instead, the state is a collection of unspent outputs from previous transactions.

When a user sends funds, they consume one or more UTXOs and create new ones, with the remainder returned to them as change. This model is highly parallelizable, as different transactions can be processed independently if they do not share the same inputs.

It also provides enhanced privacy, as users can generate new addresses for every transaction. The UTXO model is robust and well-understood, providing a clear path for auditing the total supply of a currency.

It is the foundation for many complex financial transactions, including multi-signature wallets and advanced smart contract interactions. By focusing on the flow of value rather than account states, it simplifies the logic of tracking assets across the network.

It is a cornerstone of the original vision for peer-to-peer electronic cash.