The second law of thermodynamics is commonly referred to as the Law of Increased Entropy, and it holds that energy becomes less usable over time. Therefore, while the quantity, based on the first law, of energy remains the same, the quality of energy decreases over time, based on the second law.
Usable energy is inevitably used for productivity, growth and repair. In the process, usable energy is converted into unusable energy. Thus, usable energy is irretrievably lost in the form of unusable energy.
"Entropy" is defined as a measure of unusable energy within a closed or isolated system (the universe for example). As usable energy decreases and unusable energy increases, "entropy" increases.