Crypto glossary
Weak subjectivity
Weak subjectivity is the idea that new or long-offline proof-of-stake nodes need a recent trusted checkpoint to tell the real chain from a fake one.
The term comes from a 2014 essay by Vitalik Buterin. A fully objective system lets any node, starting only from the genesis block, identify the correct chain by rules alone; proof of work works this way. Proof of stake cannot quite manage it, because of long-range attacks with old keys. It is weakly subjective: a node needs one piece of social information, a recent block hash it trusts, and from there the protocol's rules do the rest.
In practice, Ethereum consensus clients start with checkpoint sync. They download a recent finalized state from a trusted source, such as a client team's endpoint, a staking provider or a friend's node, and verify it against block explorers or other sources. The weak subjectivity period, measured in weeks or months depending on validator set size and churn, is how long a node can stay offline and still rejoin safely without a new checkpoint.
Critics see this as a step back from Bitcoin's trust model; supporters reply that users already trust whoever gave them their software. Either way, the trust involved is small and checkable, not an ongoing reliance on any authority.
Example
A home staker sets up a new Ethereum node and points it at a public checkpoint sync endpoint, then compares the finalized block root with two block explorers before letting it run.