Equilibrium is a condition in which the resultant or vector sum of all forces acting upon a particle is zero. A rigid body (by definition distinguished from a particle in having the property of extension) is considered to be in equilibrium if, in addition, the algebraic sum of the moments of the force components, both translational and rotational, along or about each of three mutually perpendicular axes chosen with respect to the body, is equal to zero. Thus, the body in equilibrium experiences neither linear acceleration nor angular acceleration and, unless disturbed by an outside force, will continue in that condition indefinitely. An equilibrium is said to be stable if small, externally induced displacements from that state produce forces that tend to oppose the displacement and return the body or particle to the equilibrium state. Examples include such systems as a pendulum or a brick lying on a level surface. An equilibrium is unstable if the least departures produce forces tending to increase the displacement. An example is furnished by a ball bearing balanced on the edge of a razor blade.
Equilibrium can also be a condition or state of a thermodynamic system, the properties of which do not change with time and that can be changed to another condition only at the expense of effects on other systems. For a thermodynamic equilibrium system with given energy, the entropy is greater than that of any other state with the same energy.
Excerpt from the Encyclopedia Britannica without permission.