In thermodynamics, equilibrium, from the Latin aequi- meaning “equal, even” + -libra meaning “weight”, is an evolution condition in accordance with time of the “state” of a system and its surroundings where a system is said to be in equilibrium when neither its state nor that of the surroundings evolves with time.  In earth-bound systems, daily heat input from the sun punctuates the state of each thermodynamic system away from equilibrium, thus forcing chemical species to evolve. 
Thermodynamic equilibrium exists, said another way, when the properties of a system are assumed constant from point to point and when there is no tendency for change with time. 
The first to use the term "equilibrium" in thermodynamics was French engineer Sadi Carnot who spoke of "equilibrium in the caloric", referring to a state of arrangement in which no heat would flow between bodies and no work could be done. In thermodynamics, jumping from Carnot to German physicist Rudolf Clausius, origin of most equilibrium theories, trace their origin to the work of American engineer Willard Gibbs, particularly his 1876 publication “On the Equilibrium of Heterogeneous Substances”  In the 1930s, the branch of nonequilibrium thermodynamics began to emerge as researchers began to apply the concepts of energy and entropy to heat transfers in fluid systems, such as in the formation of Benard cells, taking the "system" to be a small differential region of substance in the medium subjected to continuous flows of matter and energy.
In simple terms, chemical species evolve according to the logic of chemical reactions. In general, for earth-bound systems, being isothermal-isobaric systems, changes in the Gibbs free energy are useful in indicating the conditions under which a chemical reaction will occur. The spontaneity rule for reactions is that if ΔG is positive a reaction will only occur if energy is supplied to force it away from the "equilibrium" position (i.e. when ΔG = 0). If ΔG is negative the reaction will proceed spontaneously to equilibrium.  The combined set of reactions in a system characterize the equilibrium state of the system at any given time.
Equilibriums, in general, can be punctuated, stable, meta-stable, undifferentiated, unstable, near-equilibrium, far-from equilibrium, quasi-equilibrium, or non-equilibrium, among other terms. 
Difficulty on application in human systems
In human thermodynamics, there currently is great debate and confusion as to how the thermodynamic concept of equilibrium applies to systems of evolving human molecules. In the early 20th century, economists and sociologists, such as Vilfredo Pareto, began to apply Gibbs’ equilibrium criterion to the modeling of economic systems and social systems.
The general difficulty in applying the conception of equilibrium, as defined in thermodynamics, traces to the confusions between the “thermodynamic system” being defined using the standard, diurnal-time, heat contact, piston-and-cylinder, “Carnot cycle model”, verses the continuous-time, heat transfer, fluid mechanics, Bénard cells type, “hot plate model”. In loose term, the issue comes down to a debate on whether to use Gibbsian thermodynamics (1870s) or Prigoginean thermodynamics (1950s) to study the evolution of the biosphere.
In 1977, in contrast to the logic of classical thermodynamics, Belgian chemist Ilya Prigogine expressly stated that that the Gibbs' equilibrium method is not applicable to life and that evolving living systems, such as communities of people, are “far-from-equilibrium” systems.  In 1978, in opposition to Prigogine, Russian physical chemist Georgi Gladyshev proposed that evolving living systems are “quasi-equilibrium” systems.  Beyond this, others, culling from a mix of ecological thermodynamics and fluid mechanics (i.e. Bénard cells), model human systems in terms of distance from equilibrium: (a) the equilibrium state, (b) slightly away from equilibrium, (c) near-equilibrium, and (d) far-from-equilibrium. 
In modern terms, the confusion existent between most biochemistry textbooks, which model (internal bodily) biochemical reactions as Gibbs-like equilibrium reactions, and many recent physics models of human social systems (reactions between humans), which model evolution as a far-from-equilibrium Prigogine-like fluid dynamics system, has caused much mishap. Some, in recent years, being caught up in this mix of theories, have summarized the matter by stating that “life is made up of reactions in the near equilibrium realm and may not be so far from equilibrium as has been suggested.”
In terms of thermodynamic approach to equilibrium in economics, some define equilibrium such that the system, instead of evolving in time, simply changes it position in the space of macroscopic parameters, remaining on certain surface, the surface of state, singled out by the “equation of state”. 
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3. Prigogine, Ilya. (1977). "Time, Structure, and Fluctuations", Nobel Lecture (in chemistry), Dec 08
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8. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (preview). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two), (preview). Morrisville, NC: LuLu.
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● Thermodynamic equilibrium – Wikipedia.