In theoretical physics, an instanton is a type of solution to certain field equations in quantum field theory, particularly in non-abelian gauge theories and in the context of quantum chromodynamics (QCD). Instantons represent non-perturbative effects and are typically associated with tunneling phenomena in a semi-classical approximation of quantum fields.
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An instanton is a localized finite-action classical solution in Euclidean spacetime, used in a semiclassical description of quantum tunnelling and other nonperturbative effects. In one-dimensional quantum mechanics it can connect distinct degenerate potential minima over infinite Euclidean time. A quartic double-well instanton is an elementary example; a Yang-Mills instanton is a gauge-field example with different geometric structure.