A measure of the disorder of a closed system. The second law of thermodynamics says that the entropy (and disorder) increases as time moves forward. Entropy helps to explain numerous phenomena, including why it is easier to make a room dirty than to make it clean, why its more difficult to parallel park a car than to pull out of the parking spot and in which direction a chemical reaction will go.
Acknowledgements: Thank you Mike for the numerous discussions on entropy and for the intuitive examples mentioned above.