Entropy
A fundamental thermodynamic property measuring the degree of microscopic disorder, randomness, or thermal energy unavailable for mechanical work in a system.
Governing formula
dS = dQ_rev / T, S = kB ln Ω
SI unit
J/K
In depth
According to Boltzmann's statistical definition (S = kB ln Ω), entropy is proportional to the natural logarithm of the number of accessible microstates Ω. The Second Law of Thermodynamics dictates that total entropy in an isolated system can never decrease over time.
Examples in the real world
Ice melting into water increasing molecular disorder, or heat diffusing from a hot cup of coffee to the cooler surrounding air until equilibrium is reached.