A number used once, attached to a message so that the same message cannot be validly processed twice.
Why it matters
Nonces are the ordinary defence against replay. They also explain a class of stuck-transaction behaviour: because an account’s transactions must be processed in nonce order, one pending transaction can hold up every later one from the same address.
What you can check
An account’s current nonce is public and readable from any node or explorer. In signed-message schemes such as permit signatures, the nonce is part of what is signed, which is what stops one signature being presented repeatedly.
Where it showed up
Records in the Incident Tracker that turn on this: Nomad bridge replay exploit.
Not advice. Definitions are for understanding, not instruction. Nothing here is financial, legal, tax or security advice.