Nonce
A nonce is a "number used once": a counter on account-based chains, and the value miners vary while searching for a valid block hash.
In depth
On Ethereum every account has a nonce that increments with each transaction, so a signed payment cannot be re-broadcast a second time. In mining, the nonce is the field in the block header that miners change billions of times until the header’s hash meets the difficulty target. Same word, two related jobs.
Why it matters
A stuck transaction is often a nonce problem; understanding it helps you diagnose delays.