A type of blockchain protocol upgrade that is not backward-compatible, resulting in a split of the original chain.
A hard fork is a type of protocol upgrade or change in a blockchain network that is not backward-compatible. This means that nodes running the old version of the software will not recognize or correctly interpret blocks created by nodes running the new version, leading to a split in the blockchain. After a hard fork, two separate blockchains will exist: one that follows the old rules and one that follows the new rules. Hard forks can be planned (to introduce new features or fix issues) or contentious (when there is disagreement about the change). Notable examples of hard forks include the split of Bitcoin and Bitcoin Cash, and Ethereum and Ethereum Classic.
Bitcoin Cash (BCH) is an example of a hard fork from the original Bitcoin (BTC) blockchain, which occurred in 2017 due to disagreements over how to scale the network