An Etymological Dictionary of Astronomy and Astrophysics

An equilibrium state of a system in which if a small perturbation away from equilibrium is applied, the system will move farther away from equilibrium state. For example, mechanical equilibrium in which the potential energy is a maximum, as a sphere placed on top of a hill. Mathematically, if the second derivative of the energy with respect to the coordinate of interest is negative,
the system is in an unstable equilibrium. → stable equilibrium.

See also: → unstable; → equilibrium.