Mempool

Definition

A mempool is a node's local collection of transactions it has accepted for possible inclusion in future blocks. There is no single globally identical Bitcoin mempool: nodes can learn about transactions at different times and apply different relay, replacement, and memory-management policies. Transactions may enter through peers or local wallet broadcasts and leave when they are confirmed, replaced, expired, or removed under the node's rules. Being present in one mempool does not guarantee propagation across the network or inclusion by a miner. Fee rates influence selection, but miners can apply their own preferences and package considerations. A transaction may also depend on another unconfirmed transaction, making their relationship important. Mempool observations are useful for estimating congestion and fees, but they describe pending local information rather than a finalized ledger or a centrally managed queue with guaranteed ordering.