What is "aplanatic"

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.

a. (context of an optical system English) Free from, or corrected for, spherical aberration and chromatic aberration

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