Entropic force
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In physics, an entropic force acting in a system is a macroscopic force whose properties are primarily determined not by the character of a particular underlying microscopic force (such as electromagnetism), but by the whole system's statistical tendency to increase its entropy.
A standard example of an entropic force is the elasticity of a freely-jointed polymer molecule: if the molecule is pulled into an extended configuration, the fact that more contracted, randomly coiled configurations are overwhelmingly more probable (i.e. possess higher entropy) will result in the chain eventually returning (through diffusion) to such configurations. To the macroscopic observer, the precise origin of the microscopic forces that drive the motion is irrelevant: The observer simply sees the polymer contract into a state of higher entropy, as if driven by an elastic force.
Entropic forces occur in the physics of gases and solutions, where they generate the pressure of the ideal gas and the osmotic pressure of a dilute solution, and in colloidal suspensions, where they are responsible for the crystallization of hard spheres.
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