The term, “weak subjectivity” pertains to a concept that describes a requirement found on Proof of Stake (PoS) blockchains. In this context, nodes rely on each other in order to determine the present state of the system. The concept was developed by programmer, writer, and Ethereum co-founder, Vitalik Buterin.
Understanding the concepts of subjectivity and objectivity in the context of blockchains is pertinent to understanding Weak Subjectivity. These are integral concepts to a blockchain in that it presents more than one valid chain. Simply, there are multiple possible paths from the Genesis Block to the most recent block. These are all considered valid.
Being under the category of Weak Subjectivity, nodes would be able to learn which chain is active. Subjectivity is considered weak as it only occurs in two scenarios: when new nodes join the network and when nodes go offline for extended periods.