Mempool holds unconfirmed transactions accepted by a particular node under its validation and operating rules. It is not a single network queue or a promise of confirmation.
Nodes receive transactions in different orders and may use different relay policy, memory limits or available parent transactions. A consensus-valid transaction may fail local admission rules. Inclusion in a valid block is checked against consensus, regardless of earlier presence in the node’s mempool. [Bitcoin Core v29.0: transaction admission]
Confirmation, replacement, expiry or memory eviction can remove a transaction. A conflicting spend may displace the original attempt; disappearance from one node does not cancel it network-wide. After a reorganization, some disconnected transactions may return if they pass fresh checks. [Bitcoin Core v29.0: mempool]
Watch sat/vB and unconfirmed ancestors rather than a fixed queue position. Package profitability can affect miner selection. getmempoolentry describes a locally known transaction; getmempoolinfo describes the local pool. A public explorer shows its own view. Even a high rate guarantees no exact block or confirmation time. [Bitcoin Core v29.0: mempool RPC]
For the clearest picture, read this entry together with Transaction, Transaction fees, Mining, Full Node, UTXO. The reverse links also lead from Mining, Transaction, UTXO, Transaction fees.
01Is there one global Bitcoin mempool?+
No. Every node maintains its own local mempool according to what it has received, its software settings, capacity and relay policy.
02What happens if a transaction disappears from a mempool?+
It is not automatically invalid or cancelled. A node may have evicted or replaced it, while other nodes may still retain it and a miner may still confirm it.