Vitalik Buterin used the phrase “weak subjectivity” to characterize a necessity present in Proof of Stake (PoS) blockchains, where nodes must rely on other nodes to ascertain the system’s current state.
Subjectivity is the notion that there may be numerous seemingly accurate copies of a specific blockchain ledger and that it is acceptable if a machine cannot automatically select one of them. This contrasts with a blockchain, where the right ledger may be verified objectively by examining factors like which chain has the most proof-of-work while adhering to a predetermined set of validity requirements.
The concept of weak subjectivity holds that subjectivity is acceptable over lengthy time periods but unsuitable during brief ones. In particular, it is suggested that if you have a node that is constantly online, you will be able to determine the correct ledger automatically. However, if your node goes offline for a prolonged period of time, you might need to get information from another source to determine the correct ledger.
