A pentachromat is an organism for which the perceptual effect of any arbitrarily chosen light from its visible spectrum can be matched by a mixture of no more than five different pure spectral lights. The condition of being a pentachromat is called pentachromacy.
The normal explanation of pentachromacy is that the organism's retina contains five types of colour receptors with different absorption spectra. In practice the number of such receptor types may be greater than five, since different types may be active at different light intensities.
Some birds and butterflies have five or more kinds of colour receptors in their retinae, and are therefore believed to be pentachromats, though psychophysical evidence of functional pentachromacy is not easy to come by.