"Unspent transaction" refers to a financial transaction where the full amount of money involved has not yet been used or transferred. The funds remain unused and available for further transactions or purposes. Full definition
According to Wilcox, «We have a bug we inherited from the bitcoin codebase where if you verify signatures with a whole lot of [unspent transaction outputs or UTXOs], it takes too long. (coindesk.com)
When the cost of bitcoin miner fees began rising exponentially in 2017, BitPay's own operational cost of moving bitcoin payments (sweeping unspent transaction outputs, or UTXOs) increased exponentially. (blog.bitpay.com)
But from then on, its value simply becomes part of the combined Unspent Transaction Output (UTXO) set, which defines all outputs that store coins and could potentially be spent at any time. (bitcoinmagazine.com)