Glossary
Weak Subjectivity
A proof-of-stake property where a node syncing from scratch or after long downtime must trust a recent checkpoint, rather than the genesis block alone, to safely identify the canonical chain.
Weak subjectivity describes the fact that a proof-of-stake node cannot always determine the canonical chain from the genesis block and protocol rules alone. A peer that has been offline for a long time, or that is syncing for the first time, lacks a recent honest reference and could be fooled by a forged history built with old validator keys.
To stay safe, such a node must obtain a recent weak subjectivity checkpoint — a trusted block hash from a friend, client developer, or block explorer — and treat it as a starting anchor. From that point the node can apply slashing and fork-choice rules normally. Unlike proof-of-work, which is "objective" and needs only genesis, proof-of-stake accepts this mild trust assumption to defend against long-range attacks.