A state in which a transaction has not been appended to the blockchain.
Unconfirmed transactions in crypto are transactions that have not been added to the blockchain. Nodes store unconfirmed transactions in mempools. When miners include a transaction in a block, it is considered confirmed. However, receivers, like centralized exchanges, may require several confirmations before they consider a blockchain transaction safe enough to credit a deposit. Opting for a low fee is a common reason why a transaction gets stuck in the mempool and is unconfirmed.
Nodes store transactions that have been submitted by users in the mempool. Miners then choose transactions from the mempool and include them in blocks, thereby confirming them and adding them to the blockchain. The transaction cannot be reversed after that, although orphaned blocks can theoretically contain transactions that are confirmed but not accepted by most nodes.
If a transaction is unconfirmed, it means it has not yet been picked up by a miner to be added to the blockchain. In most cases, this is simply due to the fact that blockchain transactions take some time, and the chain is experiencing congestion. The more demand there is for blockchain transactions, the higher the chance that a transaction remains unconfirmed for longer than during normal times.
If a transaction is unconfirmed for an unusually long time, this may be due to a low fee that is unattractive for miners to include in a block. This can cause transactions to be unconfirmed indefinitely if the fee is significantly lower than necessary.
If a transaction on Bitcoin remains unconfirmed for longer than 24 hours, it can be canceled. At least three miners need to confirm a transaction for it to be considered confirmed.
On Ethereum, transactions can be canceled by submitting a new transaction with the same nonce and a higher gas price. In that case, miners pick up the new transaction and cancel the old transaction.