A non-backward-compatible rule change that can split participants onto incompatible chains.
Hard fork 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.
Hard fork 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 Hard fork helps distinguish a verifiable Bitcoin mechanism or historical record from slogans, products and market narratives.
For the clearest picture, read this entry together with Consensus rules, Soft fork, Full Node, Blocksize War. The reverse links also lead from Blocksize War, Roger Ver.
Linked to related atlas coordinates
Grounded in a source record
Explains function and trade-offs
Part of the open Bitcoin knowledge graph