UTXOs stand for Unspent Transaction Output. They are an important accounting method on the Bitcoin blockchain.
If you want a really in-depth explanation of UTXO's I recommend checking out Rajarshi Maitra's Medium Article 'What the heck is a UTXO'.
UTXO is essentially referring to the balance of coins held by a specific address. That is because it represents the output of a transaction received by a wallet which can be spent in the future - as it is unspent.
When building on-chain indicators, UTXOs are often used as the data sources for a lot of the data. By looking at the ages, sizes, and amounts of UTXOs on the Bitcoin blockchain we can compile the data to create indicators, as that aggregated data tells us about the behaviours of participants on the blockchain.
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