Glossary
Fork
Either a divergence in a blockchain's history (two competing chains) or a software change that creates one — soft if backward-compatible, hard if not.
"Fork" means two different things depending on context. A chain fork is a moment when two valid blocks at the same height exist; nodes follow one and orphan the other under the consensus rule.
A protocol fork is an upgrade to the node software. A soft fork tightens the rules and is backward-compatible with un-upgraded nodes; a hard fork loosens them or changes semantics, so old and new nodes will eventually disagree and the chain splits unless everyone upgrades.