Definition
A fork choice rule is a consensus mechanism component that specifies how a blockchain node selects the canonical chain when multiple valid competing chains or blocks exist. It formalizes the criteria and ordering logic used to rank candidate chains or blocks, typically based on protocol-defined metrics, and determines which branch is treated as authoritative for state transitions, validation, and finality progression.
In Simple Terms
A fork choice rule is the formal rule a blockchain uses to decide which branch of the chain counts as the real one when there are several valid options. It tells each node how to pick a single winning chain so that the network can stay in agreement about the current state.
Context and Usage
The term is used in discussions of consensus design, chain selection behavior, and protocol safety and liveness properties. It is central when analyzing how nodes react to temporary network splits, competing blocks, or reorg events, and how these behaviors interact with protocol-defined finality. Researchers and implementers reference fork choice rules when specifying node behavior and reasoning about canonical Block selection.