In
thermodynamics,
entropy (usual symbol
S) is a measure of the number of specific realizations or microstates that may realize a
thermodynamic system in a defined state specified by macroscopic variables. Entropy is commonly understood as a measure of molecular
disorder within a macroscopic system. According to the
second law of thermodynamics the entropy of an
isolated system never decreases. Such a system spontaneously evolves towards thermodynamic equilibrium, the state with
maximum entropy. Systems that are not isolated may decrease in entropy, provided they increase the entropy of their environment by at least that same amount. Since entropy is a
state function, the change in the entropy of a system is the same for any process that goes from a given initial state to a given final state, whether the process is
reversible or
irreversible. However, irreversible processes increase the combined entropy of the system and its environment.