The Collaborative International Dictionary of English v.0.48:

Achromatic \Ach`ro*mat"ic\, a. [Gr. 'achrw`matos colorless; 'a
   priv. + chrw^ma, chrw`matos, color: cf. F. achromatique.]
   1. (Opt.) Free from color; transmitting light without
      decomposing it into its primary colors.
      [1913 Webster]

   2. (Biol.) Uncolored; not absorbing color from a fluid; --
      said of tissue.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

achromatic lens
    n 1: a compound lens system that forms an image free from
         chromatic aberration