Fork
Plain English
When a blockchain splits into two separate versions. Soft forks are backwards-compatible updates (like a software update). Hard forks create a completely new blockchain (like Bitcoin and Bitcoin Cash splitting apart). Can happen due to disagreements or major upgrades.
Technical
A divergence in blockchain state occurring when: (1) temporary - multiple blocks produced simultaneously, resolved by consensus; (2) permanent - protocol rule changes creating incompatible chains. Soft forks are backwards-compatible (stricter rules). Hard forks break compatibility, requiring all nodes to upgrade. Can be contentious (community split) or planned (protocol upgrades).