In thermodynamics, entropy is an extensive state function that accounts for the effects of irreversibility in thermodynamic systems, particularly in heat engines during an engine cycle.
While the concept of energy is central to the first law of thermodynamics, which deals with the conservation of energy, the concept of entropy is central to the second law of thermodynamics, which deals with physical processes and whether they occur spontaneously.
Spontaneous changes occur with an increase in entropy.
Entropy change has often been defined as a change to a more disordered state at a microscopic level.
In recent years, entropy has been interpreted in terms of the "dispersal" of energy.