BITCOIN'S HARD-FORKS

A hard fork (or sometimes hardfork), as it relates to blockchain technology,
is a radical change to the protocol that makes previously invalid blocks/transactions
valid (or vice-versa). This requires all nodes or users to upgrade to the latest version
of the protocol software. 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
follows the new, upgraded blockchain, and the other path 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of the blockchain is outdated or irrelevant and quickly upgrade to the latest version.

BITCOIN'S HARD-FORK HERE ON OUR'S BLOG