Hard fork

A rule change old software rejects — upgrade everyone, or the network splits in two.

In plain words

A hard fork changes the rules so that old software rejects the new blocks. Upgrade together, and it is just an upgrade. Refuse, and the chain splits: two networks, two histories, two coins. Bitcoin Cash was born this way in 2017 — it loosened the block size rules, Bitcoin's nodes said no, and it left as a separate coin. Bitcoin saves hard forks for last resorts.

Why it matters

Who rules? Nodes that refuse a change keep the old coin alive.
Splits cost: confused users, divided communities, diluted names.
The power to refuse is what keeps the 21 million cap believable.

Numbers that matter

2017 — the Bitcoin Cash split, the textbook example.

Not to be confused with

Soft fork — tightens rules in a compatible way; no split required.

Go deeper

Soft fork
The blocksize war
Fork (concept)