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Mempool
memory pool, transaction pool, tx pool
The set of valid but unconfirmed transactions each full node keeps in memory, from which miners and fee estimators pull candidates.
Every Bitcoin full node validates transactions it hears on the network and keeps accepted, non-conflicting ones in its mempool until they confirm in a block or expire under local policy. There is no global mempool: each peer maintains its own view based on what it saw, in which order, and which feerate rules it applies. Wallets ask their node (or a service like mempool.space) for a snapshot of that view when they estimate fees or when they show "pending" activity to a user.
Miners and mining pools pull work from their own mempool (or from a pool template builder) when they assemble candidate blocks. Stratum V2 discussions often assume miners can see enough of the fee market to pick transactions with less blind trust in the pool operator. Policy changes such as package relay and feerate-based eviction adjust how those candidates enter and leave the set without changing consensus rules for confirmed blocks.
The mempool is where soft congestion shows up first: when demand for block space spikes, the same mempool data is what drives rising feerates for the next confirmations.