A hard fork is a major update to a blockchain's protocol that is not backward-compatible, meaning previous versions of the software will not recognize the new rules. This type of change can split the blockchain network into two separate chains if some users continue using the old software while others upgrade. Hard forks are often used to introduce new features, fix security issues, or resolve disagreements within a blockchain community. They require all participants to upgrade their software to remain part of the main chain, and can sometimes lead to the creation of a new cryptocurrency.