A fork is a change to the rules a blockchain follows. Because no single authority controls the network, upgrades happen by participants adopting new software. When everyone adopts a backward-compatible change, it is a "soft fork" and the chain carries on as one.
A "hard fork" introduces rules that are not backward-compatible. If part of the community keeps running the old software, the chain can split permanently into two separate networks, each with its own coin and its own history from the split point onward.
Forks can be routine, planned upgrades or the result of deep disagreement about a project's direction. For holders, a contentious hard fork can create a brand-new asset overnight, but it can also fragment a community and dilute attention across competing chains.
Worked example
In 2017 a hard fork split Bitcoin into the original chain and a separate one, Bitcoin Cash, each going its own way.
Related guides
This definition is general education, not investment advice. Markets — especially crypto — are volatile and you can lose money. Please read our disclaimer and see our methodology.