A unspent transaction output (UTXO) is a Bitcoin transaction output that has not yet been spent. It represents a specific amount of satoshis that can be used in a later transaction if the output’s spending conditions are met. A wallet’s spendable balance is the total of its available UTXOs, not a single balance stored in the wallet.
How a UTXO is created and spent
Bitcoin transactions use existing outputs as inputs and create new outputs. An input refers to a particular output from an earlier transaction; once spent, that earlier output is no longer available. The new transaction’s outputs may remain unspent and become UTXOs for a later transaction. The Bitcoin Developer Guide explains transaction inputs and outputs.
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For example, if an input is worth more than the amount being sent, the transaction can create one output for the recipient and another returning the surplus to the spender. That second output is called change. The transaction fee accounts for value not assigned to an output.
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An output specifies a value in satoshis and conditions for spending it. Those conditions are encoded in a script; the output is spendable only by someone able to satisfy them. The Bitcoin Developer Reference describes transaction outputs and outpoints.
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An outpoint identifies one precise output: it consists of the transaction ID and that output’s index within the transaction. The outpoint is the identifier; the UTXO is the output itself while it remains unspent.
Why a wallet balance can contain many UTXOs
A wallet’s balance is an aggregate view of the satoshis available across its UTXOs. To make a payment, the wallet selects one or more outputs as inputs, then the transaction creates outputs for the recipient and, when needed, change. As the Bitcoin Developer Guide puts it, a displayed balance represents satoshis waiting in “one or more UTXOs,” rather than a balance entry held inside the wallet.
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How Bitcoin prevents an output from being spent twice
Each transaction output can be used as an input only once. A later attempt to spend an already-consumed output is a double spend. Full nodes track currently unspent outputs in the UTXO set and use it to check whether the outpoints referenced by a transaction are still available. BIP 64 describes the UTXO set as a database used for these checks.
Coinbase outputs have a spending delay
Outputs created by coinbase transactions—the transactions that collect block rewards—cannot be spent until at least 100 blocks have been built on top of the block containing them. This maturity rule is a special timing condition; it does not change the definition of a UTXO. The Bitcoin Developer Guide covers output spending rules and coinbase maturity.
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