Hard fork is a change to blockchain rules in which blocks or transactions valid under the new rules may be invalid under the old rules.

If some participants upgrade while others continue using the old software, the network can split into incompatible histories. In other cases, participants coordinate the upgrade and the old rules cease to be used without producing a long-term split.

What a hard fork can change

A hard fork can modify block formats, transaction rules, consensus parameters, or other fundamental protocol properties. If both versions continue operating independently, they form separate networks from the point of divergence.

The term therefore describes technical rule incompatibility. It does not by itself mean that a new cryptocurrency must be created.