What Is A Hard Fork?
A Hard Fork in Bitcoin is a non-backward compatible protocol change that requires all users to upgrade. This adjustment alters Bitcoin's block structure, difficulty rules, or the set of valid transactions. While any alteration to Bitcoin's open-source code can be considered a fork, a Hard Fork makes previously invalid blocks or transactions valid. As such, older systems without the upgrade will deem the new transactions or blocks as invalid, causing a potential split in the network. To maintain consensus, upgrades during a Hard Fork are essential for all participants.