Entropy

Entropy is a thermodynamic state function that quantifies energy dispersal and the number of microscopic arrangements compatible with a system’s macroscopic state.

Boltzmann entropy: A statistical definition of entropy proportional to the logarithm of the number of microscopic states compatible with a macroscopic state. It gives the direct link between entropy and the number of accessible arrangements.
Thermodynamic state function: A property whose value depends only on a system’s state, not on the path taken to reach it. Entropy change depends on endpoints even when the actual process is irreversible.
Free energy: A thermodynamic potential that measures energy available to perform useful work under specified conditions. Free-energy changes combine energy and entropy to predict process favorability.
Enthalpy: A thermodynamic state function equal to internal energy plus pressure times volume. Unlike entropy, enthalpy tracks an energy quantity rather than multiplicity or dispersal.
Gibbs entropy: An entropy measure for a probability distribution over microscopic states, defined using the probabilities of those states. It extends the statistical account to ensembles with unequal probabilities.
Reversible process: An idealized process that can be reversed by infinitesimal changes without leaving net effects in system and surroundings. Reversible heat transfer provides the defining relation for entropy change.
Carnot cycle: An ideal reversible heat-engine cycle operating between two thermal reservoirs. Its efficiency limit follows from entropy balance between hot and cold reservoirs.
Information entropy: A measure of uncertainty in a probability distribution, defined in information theory. Its mathematical form resembles statistical entropy, but its interpretation concerns uncertainty.
Second law of thermodynamics: The law that the total entropy of an isolated system never decreases in spontaneous processes. It turns entropy change into a constraint on the direction of natural processes.
Heat: Energy transferred between systems because of a temperature difference. Entropy change for reversible transfer is heat divided by temperature.

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