CHSH inequality

Etymology
Named after, , , and , who described it in a much-cited paper published in 1969.

Noun

 * 1)  An inequality used in the proof of Bell's theorem: $$|S|\leq 2,$$, where $$S = E(a, b) - E\left(a, b'\right) + E\left(a', b\right) + E\left(a', b'\right)$$.