Fork
A change to a blockchain's protocol rules; soft forks are backwards-compatible, hard forks are not.
A fork is a change to the rules (software) that govern a blockchain. Soft forks tighten the rules in a backwards-compatible way — old nodes still accept new blocks. Hard forks broaden or fundamentally change the rules such that old and new nodes diverge; without universal adoption, the chain splits into two.
Notable hard forks include Bitcoin Cash (BCH) splitting from Bitcoin in 2017 over block-size debates, and Ethereum Classic (ETC) splitting from Ethereum in 2016 after the DAO hack reversal. Both chains continue to operate independently.
Planned hard forks are used as upgrade mechanisms — Ethereum's move to PoS (The Merge) was a hard fork that all participants agreed to adopt, resulting in no chain split.