A hard fork refers to a split in the blockchain—where Bitcoin’s historical transaction history is continuously recorded—that occurs when the validation rules specified by the protocol are relaxed.
When a hard fork occurs, nodes that have not adopted the new validation rules may reject blocks and transactions generated under those rules as invalid.
In other words, because there is a risk of inconsistency between nodes that have adopted the new rules and those that have not, discrepancies may arise in determining the longest valid chain, potentially leading to a permanent fork.
In contrast, a blockchain fork caused by changing the validation rules to be more stringent is called a soft fork.Blocks and transactions
generated based on the new validation rules are valid on all nodes, including those still using the old rules, and thus do not result in inconsistencies where some nodes reject them as invalid.
Consequently, the risk of a permanent fork is reduced, and it is expected that the network will eventually converge onto the branch with the new validation rules at some point in the near future.
