A transaction that is not included in any block. Also known as a “0-confirmation” transaction. Unconfirmed transactions are relayed by nodes and stay in their mempools . Unconfirmed transactions stay in the pool until the node decides how to dispose of it, finds it in the blockchain or includes it in the blockchain (if it is a miner ).
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc