A nonce is a value used for a particular protocol purpose, often described as a number used once. Its required behavior depends on context: it need not be random, secret or globally unique. Bitcoin mining and Ethereum transactions use the same word for different mechanisms.
Bitcoin's mining field
A Bitcoin block header contains a 32-bit nonce that miners vary while searching for a header hash at or below the target. Testing every nonce does not guarantee success. Miners can change other permitted data, including coinbase data that changes the Merkle root, to create more candidates.
Ethereum's account sequence
An ordinary Ethereum transaction carries the sending account's required nonce. Accepted transactions advance this sequence, preventing the same transaction from executing repeatedly against an unchanged sequence. A missing earlier nonce can hold later transactions back. Different pending transactions with the same sender and nonce compete for the same position, subject to replacement policies.
Check the surrounding rules
Ethereum account nonces can also advance through mechanisms such as EIP-7702 authorizations, so they are not always just a count of outgoing transactions. A nonce alone does not provide protection across different chains; chain identifiers and signature rules also matter. Mining nonces and account nonces should not be interpreted interchangeably.