What does aplanatic mean?
The definition of aplanatic
Ap`la*nat”ic, a.
Etym: [Gr. (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.