Equilibrium is a state in which no net change is occurring. Something in this state could be considered to be stable, balanced, or unchanging, and this is extremely rare, usually only existing for brief periods of time before something disturbs the balance. This concept is very important in the sciences, influencing everything from mathematics to the study of the universe, and there are a number of different types of equilibrium. In all cases, the term describes a stable state.
Most things tend to change over time, sometimes slowly and sometimes rapidly. The constant tendency to change makes it difficult to establish a state of stability, even when such a state is critically important. In the human body, for example, this is known as homeostasis, and it is very desirable — the body is constantly working to achieve it. In a simple example of how homeostasis works, cells and their surrounding fluid strive to maintain an isotonic environment, which allows waste material to flow out of a cell while new material flows in, resulting in no net change inside the cell.
In chemistry, the term “dynamic equilibrium” is sometimes used to describe a situation in which reactions and activity in a solution are balanced by opposing reactions, keeping the solution stable. In a simple example, a person can imagine placing two people on either end of a pole. If each person pushes with equal force against their end of the pole, the pole will not move, because the two opposing forces balance each other out.
It also plays a role in game theory, in the form of the symmetric equilibrium, in which all of the players in a game use an identical strategy. The field of mathematics is often very interested in the applications of this concept for everything from explaining how economic systems work to exploring complex problems and issues in physics.
Biological equilibrium is also important. Beyond the state of balance achieved in homeostasis, the body also uses the principles to maintain physical balance. The natural environment also tries to maintain a state of balance so that it can support living organisms. Biological researchers are interested in theoretical scenarios, such as what might happen if evolution reached a point of no net change, and living organisms stopped evolving. This situation is highly theoretical, because it would involve removal or counterbalancing of the numerous complex pressures that cause living organisms to evolve over time.