An Etymological Dictionary of Astronomy and Astrophysics
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Not → stable, as in → unstable atom, → unstable equilibrium.

→ un-; → stable.

An atom whose nuclei → decay by → radioactivity.

→ unstable; → atom.

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.

→ unstable; → equilibrium.