The holding area in a node for all pending transactions on the network which have not yet been confirmed in a block. Not all nodes necessarily have a mempool, and those that do don't necessarily accept all broadcast transactions into their mempool.
Miners run full nodes which collect all broadcast transactions into their mempool. The transactions with the highest fees attached to them are usually selected and confirmed in the next block, confirming them and removing the transactions from the mempool. As the found block is validated by the rest of the network each node in turn removes the blocks included transactions from their own mempool,
* All terms and definitions may update as the Cryptionary improves.