Definition
A hard fork is a change to blockchain validation rules that can make blocks acceptable under the new rules but unacceptable under the old ones. Nodes that retain the old rules may reject the changed chain, even when it has substantial mining work or validator support. If different groups continue maintaining incompatible histories, a persistent chain split can result. A hard fork does not always produce two active currencies, however; participants may coordinate an upgrade without maintaining an alternative chain. It also should not be confused with copying software into a separate development repository. Important practical questions include which rules change, how activation occurs, whether transactions can be replayed across branches, and how wallets identify the intended network. The compatibility distinction describes the change's technical effect, not whether the proposal is beneficial, legitimate, or broadly supported by users.
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