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:

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).