Mean-field theory

mean field theory (also known as self consistent field theory)

The main idea of the mean field theory is to focus on one particle and assume that the most important contribution to the interactions of such particle with its neighboring particles is determined by the mean field due to the neighboring particles.

Ignore the fluction(variance) and use the means. In some models, this is an excellent approximation; in others, a poor one. Because of its simplicity, however, it is widely used as a first step in understanding many types of physical phenomena.