Hard Fork
A permanent divergence in a blockchain's rules that creates two incompatible, separate chains.
What is Hard Fork?
A hard fork is a radical change to a blockchain's protocol that makes previously invalid blocks and transactions valid, or vice versa — and is not backward-compatible. Nodes that do not upgrade to the new rules will be rejected from the network and continue operating on the old chain, effectively splitting the blockchain into two. Hard forks can be planned upgrades with community consensus (such as Ethereum's merge to proof-of-stake), or contentious splits driven by disagreement (such as the Bitcoin Cash split from Bitcoin in 2017). When a contentious hard fork occurs, holders of the original coin typically receive an equivalent amount of the new forked coin. Hard forks differ from soft forks, which are backward-compatible rule changes that do not split the chain.
Example
In August 2017, Bitcoin underwent a contentious hard fork over how to scale the network. One group increased the block size limit from 1 MB to 8 MB, creating Bitcoin Cash (BCH). Bitcoin holders at the time of the fork automatically received an equal amount of BCH. Bitcoin (BTC) continued on the original chain, while BCH became a separate, incompatible blockchain.
Source: Investopedia — Hard Fork