A hard fork in the context of blockchain technology refers to a significant change to a network’s protocol that makes previously invalid blocks and transactions valid, or vice-versa. This kind of fork requires all nodes and users to upgrade to the latest version of the protocol software.
A hard fork is not backward compatible, meaning that nodes running the old version of the software will not recognize the new blocks created by the upgraded nodes, leading to a permanent divergence in the blockchain. This often results in the creation of two separate blockchains and cryptocurrencies: one that follows the old rules and one that follows the new rules.
Hard forks can be planned, as part of regular protocol upgrades, or contentious, leading to splits in the community. They can have significant implications for the network’s stakeholders, including developers, miners, and users, as they need to decide which version of the protocol to support.