Fork

A fork is a change or split in a blockchains rules or history. A temporary fork happens when two valid blocks appear at once and the network soon settles on one chain. A planned upgrade can be a soft fork (backward-compatible) or a hard fork (a rule change that can split the network into two chains if participants disagree).

A common misconception is that every fork creates a new coin; most are routine upgrades that do not. See crypto forks explained for how forks work.