Source : Webster's Revised Unabridged Dictionary (1913)

Unstable \Un*sta"ble\, a. [Cf. {Instable}.]
   Not stable; not firm, fixed, or constant; subject to change
   or overthrow. -- {Un*sta"ble*ness}, n. Chaucer.

   {Unstable equilibrium}. See {Stable equilibrium}, under
      {Stable}.