A hard fork is a term that originates from blockchain technology and cryptocurrency, and it refers to a significant change to the protocol of a blockchain network that makes previously invalid blocks or transactions valid, or vice-versa. This type of fork requires all nodes or users to upgrade to the latest version of the blockchain software.
In a hard fork, the changed protocol is not compatible with the older version. If not all nodes upgrade, this can result in the creation of two incompatible blockchains: one that follows the old rules, and another that follows the new rules. This scenario could potentially lead to a split in the network, creating two separate cryptocurrencies. For example, the creation of Bitcoin Cash in 2017 was the result of a hard fork from the original Bitcoin network.
Hard forks can be planned or contentious. Planned hard forks are agreed upon by the community and are designed to introduce new features or fix critical security issues. Contentious hard forks, on the other hand, are due to disagreements within the community and can result in a split in the network.