Fork

When a blockchain has multiple branches, also called “tips of the chain”, and is not sure yet which one is the correct one: $D_0$ and $D_1$ below.

graph TD
	direction LR
    A --> B --> C --> D0?
    C --> D1?

It could be possible that both $D_0$ and $D_1$ adhere to the correct STF, but the Consensus Algorithm is yet to decide which one would be the correct one. It could also be that one of them is a different/incorrect STF.

The eventual correct fork is called the canonical chain/fork.

Forks are sometimes used to coordinate upgrades to the blockchain. If the majority of the nodes upgrade to a different STF at a specific block, based on the rules of the Consensus Algorithm, this new fork will become the the canonical chain. See Hard Forks.

If all nodes actually manage to upgrade at the same block, there will in fact be no fork. But usually some nodes forget to upgrade in time.