The Collaborative International Dictionary of English v.0.48:

Achromatism \A*chro"ma*tism\, n. [Cf. F. achromatisme.]
   The state or quality of being achromatic; as, the achromatism
   of a lens; achromaticity. --Nichol.
   [1913 Webster]

WordNet (r) 3.0 (2006):

achromatism
    n 1: the visual property of being without chromatic color [syn:
         colorlessness, colourlessness, achromatism,
         achromaticity] [ant: color, coloring, colour,
         colouring]