Originally: colour blindness (in humans) in which only two of the primary colours of light can be distinguished. Also (in animals): the type of colour vision resulting from the presence of two types of colour-sensitive photopigment in the retina; an instance of this. Compare "monochromacy", "trichromasy".
The human retina normally contains photopigments sensitive to red, green, and blue light. Absence of one of these, typically occurring as an inherited condition, gives rise to one of the three types of dichromacy (protanopia, deuteranopia, and tritanopia).
Late 19th century; earliest use found in London, Edinburgh and Dublin Philosophical Magazine. From di- + -chromasy, after German Dichromasie.