A monochromat is an organism that is truly color blind. That is, the perceptual effect of any arbitrarily chosen light from its visible spectrum can be matched by any pure spectral light. The condition of being a monochromat is called monochromacy.

The normal explanation of monochromacy is that the organism's retina contains only a single kind of light receptor cell, or at least that only one kind is active at any particular level of illumination. In vertebrates, which typically have two kinds of receptors, called rods and cones, active at low and higher levels of illumination respectively, there are two kinds of monochromacy:

• rod monochromacy is the condition of having only rods in the retina. A rod monochromat will be unable to see well in normal daylight levels of illumination.
• cone monochromacy is the condition of having both rods and cones, but only a single kind of cone. A cone monochromat can have good pattern vision at normal daylight levels, but will not be able to distinguish hues.

In principle there could also be a second kind of cone monochromacy, in which the retina contains no rods, and only a single type of cone. Such an animal would be unable to see at all at lower levels of illumination, though it would have good pattern vision at normal daylight levels (though it would not be able to distinguish hues). In practice it is hard to produce an example of such a retina, at least as the normal condition for a species; there are animals (for example, many birds) with very cone-rich retinas, but they all tend to have multiple types of cones.

In cone monochromats, at low light intensities the rods and cones may be active simultaneously, allowing some degree of color discrimination. However it is unlikely that this will be functionally significant since the neural apparatus for hue discrimination would presumably not be present in an animal that was monochromatic most of the time.

It used to be confidently claimed that most mammals other than humans and our fellow primates were monochromats. In the last half-century, however, evidence of at least dichromatic color vision in a number of mammalian orders has accumulated. Two of the orders of sea mammals, the pinnipeds and cetaceans clearly are cone monochromats, since the short-wavelength sensitive cone system is genetically disabled in these animals. The same is true of the owl monkeys, genus Aotus.

Both rod and cone monochromacy occur as very rare forms of color blindness in humans. Rod monochromacy, or maskun, is the more common of the two. The majority of people described as color blind, however, are either dichromats or anomalous trichromats.