The phrase blockchain fork can describe more than one technical situation. In Bitcoin documentation, a fork can refer to competing blocks at the same height. Ethereum documentation also uses “fork” for a change to protocol rules.
Forks in the Bitcoin chain
Bitcoin’s developer guide explains that blocks produced at roughly the same time can create competing branches. Among valid competing branches, the documented chain-selection rule leads peers to follow the chain that is more difficult to recreate, while blocks on shorter stale branches are discarded by the normal process.
Forks as protocol changes
Ethereum describes forks as changes to protocol rules, often associated with planned technical upgrades. A rule change can temporarily split blocks produced under older and newer rules. Ethereum notes that disagreement can, in rare cases, result in a permanent split.
Terms that should not be mixed
A temporary competing-block event, a protocol upgrade, a hard fork and a software-code fork are not interchangeable descriptions. A fork does not necessarily create a new asset, a new network or a financial opportunity. What happens depends on the protocol and the specific event.
In short
“Fork” is context-dependent. It may describe competing chain branches or a change in protocol rules, so an explanation should name the blockchain and the meaning being used.