  Abstract Contextuality is a property of systems of random variables. The
identity of a random variable in a system is determined by its joint
distribution with all other random variables in the same context. When context
changes, a variable measuring some property is instantly replaced by another
random variable measuring the same property, or instantly disappears if this
property is not measured in the new context. This replacement/disappearance
requires no action, signaling, or disturbance, although it does not exclude
them. The difference between two random variables measuring the same property
in different contexts is measured by their maximal coupling, and the system is
noncontextual if one of its overall couplings has these maximal couplings as
its marginals.
