See Memory Pool . A technical term for a collection of unconfirmed transactions stored by a node until they either expire or are included in the main chain. When reorganization occurs, transactions from orphaned blocks either become invalid (if already included in the main chain) or moved to a pool of unconfirmed transactions. By default, bitcoin nodes dispose of unconfirmed transactions after 24 hours.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc