Mean-field theory replaces fluctuating interactions with a self-consistent average field. For example, an Ising spin feels a field determined by the average neighboring spin magnetization; a self-consistency equation approximates the interacting Ising model. Neglecting correlated fluctuations can fail near a continuous phase transition. The mean-field approximation in Landau theory is a particular use of this general framework.
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Mean-field theory (MFT) is a statistical physics and mathematical physics approach that simplifies complex many-body systems by averaging the effects of all individual particles or entities on one another. In this framework, instead of dealing with the complicated interactions of every particle in a system, the average effect of all particles is considered to define a "mean field" that influences each particle.