Equilibrium or balance is any of a number of related concepts in the sciences and social sciences. In general, a system is said to be in a state of equilibrium if all influences on the system are cancelled by the effects of others. A related concept is stability; an equilibrium may or may not be stable.

Some specific examples are:

• Chemical equilibrium, the state in which a chemical reaction proceeds at the same rate as its reverse reaction, resulting in no net change in the amount of each compound.
• Mechanical equilibrium, also known as static equilibrium, the state of a body at rest or in uniform motion in which the sum of all forces acting on the body equals zero.
  • Physical balance of humans and animals is maintained with the aid of the sense of balance, and in special cases with a balance beam.
  • See also the weighing instrument balance.
• Thermodynamic equilibrium, the state of a system in which its internal processes cause no net change in its macroscopic properties (such as temperature and pressure).
• In economics, static equilibrium and general equilibrium
• Nash equilibrium in game theory, an optimum strategy for all players in a game, in the sense that no one player can benefit by changing his strategy while all other players keep theirs the same.
• Reflective equilibrium in ethics, a state in which the consequences of one's general principles are consistent with one's opinions about individual cases.
• For individuals and organisations a balance between income and expenses is often important, especially in the long run.
• Psychologically some balance between desires and satisfaction is important; somewhat paradoxically complete satisfaction may not be ideal, it can be argued that perhaps it is better if things are left to be desired.
• In various practical matters an equilibrium is useful, e.g.:
  • in a conversation, between talking and listening;
  • in a personal relationship, between giving and taking;
  • between buying and reading books (apart from lent books).

An addiction is any of various forms of unbalanced behavior.

In electricity, a balanced signal is also called a differential signal.

Further Reading

Mechanical equilibrium:
Marion & Thornton, Classical Dynamics of Particles and Systems. Fourth Edition, Harcourt Brace & Company (1995).
Thermodynamic equilibrium:
F. Mandl, Statistical Physics, Second Edition, John Wiley & Sons (1988).
Nash equilibrium:
A. Mehlmann, The Game's Afoot! Game Theory in Myth and Paradox, Friedr. Vieweg & Sohn (1997).