[eip]

According to the second law of thermodynamics the entropy of an isolated system never decreases; such a system will spontaneously evolve toward thermodynamic equilibrium, the configuration with maximum entropy. Systems that are not isolated may decrease in entropy.

http://en.wikipedia.org/wiki/Entropy

An isolated system obeys the conservation law that its total energy–mass stays constant.

Because of the requirement of enclosure, and the near ubiquity of gravity, strictly and ideally isolated systems do not actually occur in experiments or in nature. They are thus hypothetical concepts only.

http://en.wikipedia.org/wiki/Isolated_system