A rule change that can accept blocks older software would reject.
Hard fork meaning
A hard fork broadens or otherwise changes consensus validity in a way that is not backward-compatible for nodes enforcing the old rules. If participants follow incompatible rules, the network can split into separate histories and assets.
How it works in Bitcoin
Upgraded nodes may accept a block that old nodes consider invalid. Coordination matters because accumulated proof of work does not cause old nodes to accept a block that violates their rules.
A simple example
If a new rule permits a transaction form banned by old software, a block using it can create a lasting split unless the old network also changes.
Why Hard fork matters
Network terms explain how independently operated software shares, validates and orders information. They help separate a convenient data viewer from the rules a node actually enforces.
What people often get wrong
The words hard fork do not say whether a proposal is wise, popular, or likely to succeed. They describe compatibility, not quality.
Quick checks before using the term
- Which actor performs this job?
- Is the behavior a consensus rule or an implementation choice?
- What evidence can a node verify independently?
Apply these questions to the concrete example above. A precise answer is more useful than repeating the definition without naming the actor, rule or failure path.
Check the primary reference
This entry is written in plain English, but the technical source is available when you need exact protocol detail.
Bitcoin Developer Guide: Block Chain ↗