A node's local collection of valid, unconfirmed transactions.
Mempool meaning
Mempool is short for memory pool. Each node maintains its own set under its validation and relay policies, so there is no single global Bitcoin mempool. A transaction can appear in one node's pool and not another's.
How it works in Bitcoin
Nodes remove transactions when they confirm, conflict with a confirmed spend, expire, are evicted under memory pressure, or are replaced under policy. Miners commonly use fee rate and package relationships when selecting transactions for a candidate block.
A simple example
During heavy demand, higher-fee-rate transactions may confirm first while lower-fee transactions wait. The payment remains unconfirmed even if many explorers display it.
Why Mempool matters
Transaction terms matter whenever bitcoin moves. Understanding them reduces mistakes involving fees, confirmation status, change outputs and irreversible destinations.
What people often get wrong
Mempool acceptance is not consensus approval for a future block. Relay policy can be stricter than consensus rules, and local pools differ.
Quick checks before using the term
- Is the transaction only broadcast or already confirmed?
- Which outputs and fee are being created?
- Can wallet policy replace or delay it?
Apply these questions to the concrete example above. A precise answer is more useful than repeating the definition without naming the actor, rule or failure path.
Check the primary reference
This entry is written in plain English, but the technical source is available when you need exact protocol detail.
Bitcoin Developer Guide: Transactions ↗