Unstable equilibrium

from The Collaborative International Dictionary of English v.0.48

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.
   [1913 Webster]

   {Unstable equilibrium}. See {Stable equilibrium}, under
      {Stable}.
      [1913 Webster]