Source : Webster's Revised Unabridged Dictionary (1913)

Aplanatic \Ap`la*nat"ic\, a. [Gr. 'a priv. + ? disposed to
   wander, wandering, ? to wander.] (Opt.)
   Having two or more parts of different curvatures, so combined
   as to remove spherical aberration; -- said of a lens.

   {Aplanatic focus} of a lens (Opt.), the point or focus from
      which rays diverging pass the lens without spherical
      aberration. In certain forms of lenses there are two such
      foci; and it is by taking advantage of this fact that the
      best aplanatic object glasses of microscopes are
      constructed.

Source : WordNet®

aplanatic
     adj : free from or corrected for spherical aberration; "an
           aplanatic mirror"