Definition of Accessibility relation

Accessibility relation is a binary relation R between possible worlds which has very powerful uses in both the formal/theoretical aspects of modal logic as well as in its applications to things like epistemology, metaphysics, and value theory. In informal terms, it is the notion that modal statements (it is possible that x, it is necessary that x) may not take the same truth value in all possible worlds, or in other words that the set of possible worlds varies depending on which possible world you are in, i.e. from any given possible world, some other possible worlds may be accessible, and others may not be.