What is Hard fork?
A rule change that loosens what is valid, so old nodes reject the new blocks and the chain splits unless everyone upgrades.
A hard fork requires universal adoption to avoid producing two chains. Where adoption is contested, the result is a permanent split with two assets, as happened in 2017.
Bitcoin has not hard-forked its consensus rules since its early years, which is a deliberate outcome of the norms around changing them.
Related terms
Soft fork
A rule change that tightens what is valid, so old nodes still accept new blocks.
NetworkConsensus rules
The validity conditions every node enforces: issuance schedule, signature validity, block weight, and…
NetworkNode
A computer running Bitcoin software that independently validates every block and transaction against the…
This definition is part of the Bitcoin Logical glossary. For the fuller explanation, start with Learn, or see the live numbers on The Bitcoin Signal.