Source : Webster's Revised Unabridged Dictionary (1913)

Astatic \A*stat"ic\, a. [Pref. a- not + static.] (Magnetism)
   Having little or no tendency to take a fixed or definite
   position or direction: thus, a suspended magnetic needle,
   when rendered astatic, loses its polarity, or tendency to
   point in a given direction.

   {Astatic pair} (Magnetism), a pair of magnetic needles so
      mounted as to be nearly or quite astatic, as in some
      galvanometers.

Source : WordNet®

astatic
     adj : not static or stable