Technical Definition

Mempool

A mempool is a node's temporary pool of valid, unconfirmed transactions waiting to be included in a block.

By Crypto University Editorial
TransactionConfirmationGas fee

✦ Key Insight

Mempools explain why transactions can remain pending, be replaced, or be observed before confirmation. Publicly visible pending transactions can also create opportunities for front-running and other MEV strategies.

✕ Common Misconceptions

There is no single universal mempool shared identically by every node. Private order flow, local policies, propagation delays, and alternative account-abstraction mempools can change what different participants see.

Detailed Explanation

When a blockchain transaction is broadcast, it may first sit in the transaction pool of participating nodes. Nodes can have slightly different views because the mempool is not a single global database. Block builders or validators select transactions from available pools according to network and fee rules.

In Practice

“A user submits an Ethereum swap with a low fee during congestion. The transaction can remain pending in nodes' transaction pools until it is included, replaced with a higher-fee transaction, or dropped.”

Dig Deeper

Confirmation

A confirmation indicates that a transaction has been included in a block and has gained additional blockchain history after it.

Transaction

A blockchain transaction is a signed or authorized instruction that changes blockchain state or transfers value when accepted by the network.

Nonce (account)

An account nonce is a sequence value used to order transactions from an account and help prevent the same signed transaction from being replayed repeatedly.

MEV

MEV is value that can be extracted by strategically including, excluding, or ordering transactions around normal blockchain activity.

Gas Fee

A gas fee is the network fee paid for computation and transaction execution on blockchains that use a gas-based fee model.