For questions about the phenomena related to correlations among distant observers. For example, this tag should be used for questions about Bell or CHSH inequalities, or for questions about quantum steering.

The phenomenon by which measurements made at a microscopic level can contradict a collection of notions known as "local realism" that are regarded as intuitively true in classical mechanics. Some observations about the correlations among multiple parties, while correctly predicted by quantum mechanics, can be proved to be impossible to achieve via local hidden variable theories.