fork
A fork is a change to Bitcoin's protocol rules. Soft forks tighten the existing rules and remain backward-compatible. Hard forks introduce new rules that older software rejects, potentially splitting the network into two separate chains.
A fork occurs when the rules of the Bitcoin protocol are changed through a software update. Not all forks are equal. A soft fork introduces changes that are backward-compatible: nodes running older software still consider blocks produced under the new rules to be valid, even if they do not enforce every detail of them. SegWit, activated in 2017, is a well-known example. It changed how transaction data is structured but allowed older nodes to continue operating without upgrading.
A hard fork is more disruptive. It introduces rules that older software considers invalid. If a significant portion of the network does not upgrade, the blockchain splits into two separate chains: one following the old rules and one following the new. Both chains share the same transaction history up to the point of the split and diverge permanently from that moment onward. Anyone who held bitcoin before the split holds coins on both chains.
Forks are ultimately decided by the economic majority of nodes, miners, users, and businesses. A proposed change that fails to win broad adoption simply does not activate. This is why Bitcoin has remained stable at its core despite years of debate: changing the rules requires convincing a decentralized, global network to voluntarily upgrade. That bar is intentionally high.