In thermodynamics, equilibrium thermodynamics is the study of systems that are at equilibrium (equilibrium state), near-equilibrium, or approaching equilibrium.

The term equilibrium thermodynamics is often used so as to distinguish between assumed continuous equilibrium states, e.g. as in geological thermodynamic studies of rock formations, verses nonequilibrium thermodynamics, where continuous inputs of energy or matter to systems acts to keep such systems in nonequilibrium states, e.g. as in Benard’s cell formation. The criterion of “local equilibrium” is often used to justify the use of equilibrium thermodynamic type equations.

Terminology
In his 1983 textbook Equilibrium Thermodynamics, English physicist Clement Adkins defines his book as a standard treatise on “classical thermodynamics”, which implies that the term “equilibrium thermodynamics” simply means standard thermodynamics applied to a particular subject, in larger part. [1] In short, equilibrium thermodynamics is a rather ill-defined term, similar to classical thermodynamics, but is essentially a synonym for basic thermodynamics.

References
1. Adkins, Clement J. (1983). Equilibrium Thermodynamics. Cambridge University Press.

Further reading
● Denbigh, Kenneth. (1981). The Principles of Chemical Equilibrium. Cambridge University Press.
● Anderson, Greg M. and Crerar, David A. (1993). Thermodynamics in Geochemistry: the Equilibrium Model.

External links
● Equilibrium thermodynamics – Wikipedia.