Hardfork

Hardfork:-

A hardfork is change in bitcoin protocol that makes invalid blocks/transactions invalid, and therefore requires all users to upgrade.

Put differently, a hard fork is a permanent divergence from the previous version of the blockchain, and nodes running previous versions will no longer be accepted by the newest version. This essentially creates a fork in the blockchain, one path which follows the new, upgraded blockchain, and one path which continues along the old path. Generally, after a short period of time, those on the old chain will realize that their version of the blockchain is outdated or irrelevant and quickly upgrade to the latest version.

A hard fork involves splitting the path of a blockchain by invalidating transactions confirmed by nodes that have not been upgraded to the new version of the protocol software.

Source:- Investopedia