The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

aplanatic
    adj 1: free from or corrected for spherical aberration; "an
           aplanatic mirror"