Term
Mempool
The mempool is the waiting area of a blockchain network — the set of valid but unconfirmed transactions that nodes have received and are holding until a miner or validator includes them in a block.
The mempool (memory pool) is a per-node cache of transactions that have been broadcast to the network but not yet included in a block. Every full node keeps its own mempool; when a miner or validator produces the next block, it picks transactions from its mempool — usually the ones offering the highest fees — and includes them.
The mempool matters when the network is congested. If demand for block space exceeds supply, transactions with low fees stay in the mempool for hours or even get dropped when nodes evict them. Bumping the fee (Replace-By-Fee on Bitcoin, or a new transaction with the same nonce on Ethereum) is how you get an unconfirmed transaction to jump ahead.
Public mempool viewers exist for most chains — mempool.space for Bitcoin, Etherscan's "Pending" view for Ethereum. They show the current fee market and how deep in the queue any specific transaction sits.
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