AdS/CFT correspondence
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For the relation of the AdS/CFT correspondence to the general context of string theory, see String theory.
In physics, the AdS/CFT correspondence (anti-de-Sitter space/conformal field theory correspondence), sometimes called the Maldacena duality, is the conjectured equivalence between a string theory defined on one space, and a quantum field theory without gravity defined on the conformal boundary of this space, whose dimension is lower by one or more. The name suggests that the first space is the product of anti de Sitter space (AdS) with some closed manifold like sphere, orbifold, or noncommutative space, and that the quantum field theory is a conformal field theory (CFT).

An example is the duality between Type IIB string theory on AdS5 × S5 space (a product of five dimensional AdS space with a five dimensional sphere) and a supersymmetric N=4 Yang-Mills gauge theory (which is a conformal field theory) on the 4-dimensional boundary of AdS5. It is the most successful realization of the holographic principle, a speculative idea about quantum gravity originally proposed by Gerard 't Hooft and improved and promoted by Leonard Susskind.

The AdS/CFT correspondence was originally proposed by Juan Maldacena in late 1997. Important aspects of the correspondence were given in articles by Gubser, Klebanov and Polyakov and by Edward Witten. The correspondence has also been generalized to many other (non-AdS) backgrounds and their dual (non-conformal) theories. In about five years, Maldacena's article had 3000 citations and became one of the most obvious conceptual breakthroughs in theoretical physics of the 1990s, providing stark new insight into both quantum gravity and QCD.