from
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]