Definition
A hard fork loosens or fundamentally changes the protocol rules in a way that old nodes cannot accept. Non-upgraded nodes will reject blocks following the new rules, causing the network to split if not all participants upgrade. If both chains maintain hashrate, two separate cryptocurrencies can result.
Bitcoin Cash (BCH) is the most notable example of a Bitcoin hard fork, created in 2017 over disagreements about block size. Bitcoin’s conservative approach to protocol changes means hard forks are extremely rare and contentious.
In Simple Terms
A non-backward-compatible protocol change requiring all nodes to upgrade. Can permanently split the chain.
A protocol change that is not backward-compatible. All nodes must upgrade or risk following a different chain. Can result in a permanent chain split.