In thermodynamics, equilibrium state or state of equilibrium is a conjunction term describing the state of a system that is in equilibrium, meaning that neither it nor its surroundings are evolving with time. The process of changing a system from one equilibrium state to another generally centers on the meaning or effect of the entropy change ΔS for the process. 
The detailed understanding of the “state of equilibrium” and the various "criterions" for this condition, as system influences change, such as with particle (chemical species) movements in and out of the system, gravitational effects, osmotic effects, electromotive force effects, etc., were laid out in the 1876 publication On the Equilibrium of Heterogeneous Substances by American engineer Willard Gibbs. The term “state of equilibrium” was first mentioned in the opening paragraph to his abstract, where he notes: 
“It is an inference naturally suggested by the general increase of entropy which accompanies the changes occurring in any isolated material system that when the entropy of the system has reached a maximum, the system will be in a state of equilibrium.”
On this logic, in his dense treatise, Gibbs extends the results of this principle, i.e. the second law of thermodynamics as defined by German physicist Rudolf Clausius, to all varieties of systems.
1. Anderson, Greg, M. and Crerar, David A. (1993). Thermodynamics in Geochemistry - the Equilibrium Model (pg. 90). New York: Oxford University Press.
2. Gibbs, Willard. (1876). "On the Equilibrium of Heterogeneous Substances", Transactions of the Connecticut Academy, III. pp. 108-248, Oct., 1875-May, 1876, and pp. 343-524, may, 1877-July, 1878.