A major change in the Bitcoin protocol that all miners and validators must implement in order to participate. Bitcoin Cash was a hard fork of Bitcoin, and Bitcoin Cash later experienced two hard forks. See
Bitcoin Cash.
Contrast with a Bitcoin "soft fork," which is an upgraded system that is compatible with the older one. For example, Segregated Witness (SegWit) and Taproot are soft forks of Bitcoin. See
SegWit,
Taproot and
Ethereum hard fork.