Hard Fork
A backward-incompatible rule change that forces every node to upgrade, sometimes splitting one chain into two.
A hard fork loosens or rewrites the rules so that old software no longer accepts the new blocks. Every node has to upgrade to stay on the network. If part of the community refuses, the chain permanently splits into two.
That's how Ethereum and Ethereum Classic parted ways in 2016 after the DAO hack. Holders of the original coin typically receive the new one too, which is part of why contentious forks get heated.
Related terms
A split in a blockchain's history or rules, whether a brief accident or a deliberate protocol change.
Soft ForkA backward-compatible rule tightening where upgraded nodes enforce stricter limits and old nodes still follow along.
BlockchainA shared digital ledger that records transactions in linked batches, copied across many computers and hard to alter.
NodeA computer running a blockchain's software, keeping a copy of the ledger and relaying transactions to peers.