Nonce expiration refers to the time limit attached to a nonce, which is a number used only once in a blockchain transaction or mining process. Each nonce is tied to a specific action, such as creating a block or signing a transaction, and ensures that the action can only be carried out once.When a nonce expires, it means that it can no longer be used for the original purpose it was assigned to. This helps prevent replay attacks, where an old transaction is maliciously reused by an attacker. Nonce expiration also encourages fresh transactions, ensuring that the network maintains accuracy and integrity.Different protocols may set varying time frames for nonce expiration. If a nonce expires before it is confirmed, users must generate a new nonce for their transaction or block, leading to additional steps in the process. This mechanism ultimately supports a more secure and efficient environment, promoting not only the validity of transactions but also the overall health of the network.