Miner-controlled coinbase data changed to create new merkle roots when the 32-bit block-header nonce space is exhausted or partitioned.
Extra nonce is best understood as part of a system rather than as an isolated definition. Its related coordinates show the mechanisms, incentives and historical records that give the term practical meaning.
Extra nonce sits between protocol rules, economic incentives and software operated by independent participants. In Bitcoin, no component is authoritative by itself: miners propose history, fully validating nodes enforce validity, and users decide which rules and software they accept.
Understanding Extra nonce helps distinguish a verifiable Bitcoin mechanism or historical record from slogans, products and market narratives.
For the clearest picture, read this entry together with Nonce, Coinbase transaction, Block template.
Linked to related atlas coordinates
Grounded in a source record
Explains function and trade-offs
Part of the open Bitcoin knowledge graph