An unconfirmed transaction refers to a cryptocurrency transaction that hasn’t been validated or processed in time to be entered into the blockchain.
Each cryptocurrency transaction has to be confirmed a set amount of times within a certain time frame for the transaction to be ‘mined’ into the chain. For example, bitcoin transactions must receive three confirmations from miners before it is considered a valid transaction.
But bitcoin transactions can sometimes fail and become ‘stuck.’ This failure to confirm can take place for various reasons, such as failed fee payment, a sudden burst in network traffic, or conversely a sudden drop in network transactions. Given the interconnected decentralized nature of blockchain technology, a sudden drop in users effectively means there wouldn’t be enough miners on the block to confirm the transactions. In the event of an unconfirmed transaction, the coins in transit come to rest in the nodes ‘Mempool’
A mempool functions as the ‘waiting room’ for a specific node’s unconfirmed transactions. Unconfirmed transactions can be stored for up to 72 hours before the network scrubs the transaction from its mempool. At this point, the individual is free to attempt to send the transaction again for a nominal fee.
Mempools are important to blockchain functionality as they allow a period of holding for transactions that have suffered some kind of disturbance. Nodes communicate to each other amongst the block and through this communication create cohesion along with the blockchain. Each node’s mempool creates a buffer period for unconfirmed transactions, giving time enough for nodes to communicate the necessary data to stabilize the chain.