Entropy

Entropy Entropy is a measure of the disorder or a measure of progression towards maximum disorder, called thermodynamic equilibrium, which signifies the number of ways in which a thermodynamic system can be arranged. In accordance to the second law of thermodynamics the entropy in an isolated system never decreases because isolated systems spontaneously evolve toward the state of maximum entropy which is called thermodynamic equilibrium.

http://upload.wikimedia.org/math/7/0/a/70adfaa8b72ecb33eabf5a17da159d1c.png

This is the original definition of entropy, and is also the macroscopic definition because it doesn't take into account the microscopic components of the system. In this equation S is the entropy of the system, T is the thermodynamic temperature of a closed system and dQ is a transfer of heat into the system that is reversible, hence the rev.