An imaginary cave in which prisoners are kept in such a way that all they can see are shadows created by puppeteers behind them, used by Plato (Republic VII.) as an allegory to explain the relationship between our perception of reality and the realm of ideas or forms, the awareness of which is made possible by an ‘escape’ into the light of intellectual understanding. Hence allusive: something likened to Plato's cave, especially a situation in which a person experiences something merely by observation, or which appears unreal.
Late 17th century; earliest use found in Robert Dixon (?1615–1688), Church of England clergyman. From the genitive of the name of Plato + cave.