Aplanatic (&unr_;), a. [Gr. 'a priv. + &unr_; disposed to wander, wandering, &unr_; 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]