UTXO
Unspent Transaction Output
A discrete chunk of spendable bitcoin. Bitcoin balances are the sum of a wallet's UTXOs, and quantum exposure is a property of individual UTXOs rather than of a wallet as a whole.
Related terms
Receiving funds repeatedly at an address that has already spent. Because the first spend publishes the public key, every later balance at that address sits behind an already-exposed key.
Whether an address's full public key is visible on-chain. Exposure matters because quantum risk to a wallet begins only once the public key is published — before that, an attacker sees just a hash.
The legacy Bitcoin address type beginning with 1, which locks funds to the hash of a public key rather than the key itself. An unspent P2PKH output does not reveal its public key.