Definition
An unconfirmed transaction is one that nodes have validated and accepted into their mempools but that no miner has yet placed into a block. It is fully signed and ready to be mined; it simply has not been selected. Until a block includes it, the transaction has zero confirmations and is not considered settled.
Why a transaction stays unconfirmed
The most common reason is an insufficient fee rate relative to mempool competition. Miners build blocks from the highest-paying transactions, so an underpaying transaction waits until either congestion eases or it is fee-bumped. Other causes include broadcasting during a congestion spike or depending on a parent transaction that is itself unconfirmed, which holds the child back until the parent clears.
What you can do about it
An unconfirmed transaction is not lost. If it was flagged as replaceable, the sender can rebroadcast it with a higher fee; if it has a spendable change output, a higher-fee child can pull it through. If a transaction underpays badly, many nodes will eventually evict it from their mempools after roughly two weeks, at which point the inputs are spendable again and the funds were never actually moved.
For resolution paths and timing, see transaction accelerator and confirmation time.
In Simple Terms
An unconfirmed transaction is one that nodes have validated and accepted into their mempools but that no miner has yet placed into a block. It…