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Gauge fields like the photon field or gluon fields are usually described using gauge potentials. An alternative is to use Wilson lines. In contrast to gauge potentials, Wilson loops are completely gauge invariant. The set of all Wilson loops is sufficient to reconstruct the corresponding gauge potential.
The formulation of Yang-Mills theory which makes use of Wilson loops is called loop formulation.
The confining phase is usually defined by the behavior of the action of the Wilson loop, which is simply the path in spacetime traced out by a quark–antiquark pair created at one point and annihilated at another point. In a non-confining theory, the action of such a loop is proportional to its perimeter. However, in a confining theory, the action of the loop is instead proportional to its area. Since the area is proportional to the separation of the quark–antiquark pair, free quarks are suppressed. Mesons are allowed in such a picture, since a loop containing another loop with the opposite orientation has only a small area between the two loops. https://en.wikipedia.org/wiki/Color_confinement