The 'waiting room' of unconfirmed transactions on a blockchain network - transactions broadcast to the network but not yet included in a block.
When a user broadcasts a blockchain transaction, it enters the mempool (memory pool), a temporary holding area maintained by each node independently. Miners (PoW) or validators (PoS) select transactions from the mempool for inclusion in the next block, typically prioritising those with the highest fee-per-unit-of-computation offered. Transactions with fees below the current market rate may remain in the mempool for minutes, hours, or indefinitely until mempool congestion clears or the user cancels and rebroadcasts with a higher fee.
Mempool data is publicly readable, which creates several trading-relevant dynamics. MEV (Maximal Extractable Value) bots scan the mempool for profitable transactions - if a large swap is pending, a bot can front-run it (insert an identical trade ahead in the same block at a higher gas fee), execute the swap, and profit from the price impact. This is particularly prevalent in DeFi and is one reason sophisticated DEX traders use private mempools (Flashbots Protect) that route transactions directly to validators, bypassing public mempool exposure.
Mempool size and fee data are available in real time from Etherscan Gas Tracker (Ethereum) and mempool.space (Bitcoin). Monitoring mempool congestion allows traders to time transactions more cheaply and understand why a transaction has not yet been confirmed.
Worked Example
A trader wants to claim a token airdrop the moment its smart contract goes live at 9:00 AM UTC. Anticipating congestion, they set their gas priority fee at 30 gwei above the current base fee of 25 gwei (total: 55 gwei effective). When the contract opens, the mempool fills with thousands of transactions all competing for block space. Their elevated tip means their transaction is included in the first post-launch block. A competitor who set only 5 gwei priority fee waits 12 minutes for inclusion - potentially missing early claiming windows if the supply is limited.