What does achromatic mean?
The definition of achromatic
Ach`ro*mat”ic, a.
Etym: [Gr. achromatique.]

1 (Opt.)
Free from color; transmitting light without decomposing it into its primary colors.
2 (Biol.)
Uncolored; not absorbing color from a fluid; — said of tissue. Achromatic lens (Opt.), a lens composed usually of two separate lenses, a convex and concave, of substances having different refractive and dispersive powers, as crown and flint glass, with the curvatures so adjusted that the chromatic aberration produced by the one is corrected by other, and light emerges from the compound lens undecomposed.

— Achromatic prism. See Prism.
— Achromatic telescope, or microscope, one in which the chromatic aberration is corrected, usually by means of a compound or achromatic object glass, and which gives images free from extraneous color.