The mempool is a shortened term for “Memory Pool,” referring to a transaction that is in the memory of a blockchain node but has not been committed to the blockchain in a block yet.
You can think of the mempool as a waiting place or queue of transactions before each is included in the blockchain. A transaction in the mempool is waiting for a node to grab the transaction and include it in a block.
The term originally came from the Bitcoin ecosystem, and different blockchain nodes have different names for it.
In the world of Bitcoin, a transaction in the mempool waits for a node to mine a block with the transaction included. In Ethereum, a transaction in the mempool waits for a node to validate it and include it in a block.
For a deeper dive into mempools, how they work, the mechanics involved, and how they can be used, see Cyfrin’s blog post “What is a Blockchain Mempool and How Does it Work?“
