Hard Fork, Hard-Forking Change

This definition comes from the technical glossary.

Definition

A permanent divergence in the block chain, commonly occurs when non-upgraded nodes can’t validate blocks created by upgraded nodes that follow newer consensus rules.

Fork (a regular fork where all nodes follow the same consensus rules, so the fork is resolved once one chain has more proof of work than another)
Soft fork (a temporary divergence in the block chain caused by non-upgraded nodes not following new consensus rules)
Software fork (when one or more developers permanently develops a codebase separately from other developers)
Git fork (when one or more developers temporarily develops a codebase separately from other developers