Unconfirmed is a status label, and the transactions it describes are covered in depth under zero confirmation transaction. This page explains the single word as it appears in wallets, block explorers and exchange deposit screens.
A transaction is unconfirmed from the moment it is signed and sent to the network until a miner or validator places it in a block. During that window it usually sits in the mempool, the waiting area of pending transactions that each node keeps. Once it lands in a block it has one confirmation, and every later block adds another; the count is explained under confirmations.
The label matters because an unconfirmed payment can still fail or be replaced. If the fee is too low it may wait for hours or drop out of the mempool, and on some networks the sender can rebroadcast a version with a higher transaction fee that pays the funds elsewhere. That is the opening for double spending, which is why exchanges and merchants usually credit a deposit only after a set number of confirmations.
How long the status lasts depends on network traffic, the fee offered and the chain's block time: seconds on fast networks, typically ten minutes or more on Bitcoin.