3D mirror symmetry
In theoretical physics, 3D mirror symmetry is a principle, or duality, that proposes a surprising equivalence between two different three-dimensional quantum field theories. The two theories appear to describe different physics, but are in fact identical. This is like having two different instruction manuals that result in building the same object; a difficult step in one manual might correspond to an easy step in the other.
The duality is a powerful tool because a problem that is very difficult to solve in one theory may be simple to solve in its "mirror" version. Specifically, this symmetry applies to three-dimensional gauge theories with a property known as supersymmetry (meaning they have eight supercharges). It is a version of mirror symmetry that relates the moduli spaces of these theories.
The principle was first proposed by Kenneth Intriligator and Nathan Seiberg in 1996. They showed that for a pair of mirror theories, the moduli space of each theory is swapped. Specifically, what is known as the Coulomb branch of one theory is the Higgs branch of the other, and vice versa. This relationship was soon given a physical interpretation in string theory by Amihay Hanany and Edward Witten, who demonstrated that it is a consequence of S-duality in type IIB string theory.