All terms
The waiting room where broadcast transactions sit before being included in a block.
When you send a transaction it does not go straight into a block. It sits in the mempool, a public queue that every node maintains, until a validator picks it up. Validators generally choose the most profitable transactions first.
Because the mempool is public, anyone can see your pending transaction before it executes. That visibility is what makes front running and sandwich attacks possible.
If your transaction is stuck, it is sitting in the mempool with a fee too low to be attractive. You can usually replace it with a higher fee version.
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