dichromatism

from WordNet (r) 3.0 (2006)

dichromatism
    n 1: a deficiency of color vision in which the person can match
         any given hue by mixing only two other wavelengths of light
         (as opposed to the three wavelengths needed by people with
         normal color vision) [syn: {dichromacy}, {dichromatism},
         {dichromatopsia}, {dichromia}, {dichromasy}]

from The Collaborative International Dictionary of English v.0.48

Dichromatism \Di*chro"ma*tism\, n.
   The state of being dichromatic.
   [1913 Webster]