Entropy is a fundamental thermodynamic property that measures the degree of disorder or randomness in a system. Introduced by Rudolf Clausius in 1865, entropy quantifies the amount of thermal energy in a system that is unavailable for doing useful work. The Second Law of Thermodynamics states that the total entropy of an isolated system can only increase over time, which explains why many natural processes are irreversible.

In practical terms, entropy helps engineers understand energy efficiency, predict the direction of spontaneous processes, and design more efficient thermal systems. Higher entropy means more disorder and less available energy for work, while lower entropy indicates a more ordered state with greater potential for useful work.