The concept of equilibrium is described in 'The Nature of Science' by James Trefil:


Equilibrium is the state of a system when the forces acting on it are in balance. Equilibrium can be stable, unstable, or neutral.

It can be applied to any system, from a planet circling its star to the population of fish in a tropical lake. In mechanics, a system is in equilibrium if all the forces on it balance, canceling one another out. If you are sitting in a chair and reading this book, for example, the downward force of gravity on your body is being canceled by the upward force exerted on you by the chair. You neither fall nor rise - you are in equilibrium.

Consider, for example, the relation between predator and prey in an ecosystem. The numbers of predator and prey will come to an equilibrium - so many foxes for so many rabbits, for example. If for some reason the equilibrium number of prey is disturbed (e.g., by rabbits having a successful breeding season) then the number of predators will increase and drive the rabbit population down again. Although these aren't physical forces, they are still forces, in a broader sense of the term, operating in the ecosystem to drive it back toward equilibrium following a disturbance.