Achromatopsia is a severe form of colorblindness where subjects experience no or very limited color vision. Causes are diverse, affecting any part of the Phototransduction Pathway from the Photoreceptors to the Visual Cortex.
Achromatopsia can be complete, where color vision is totally absent, or incomplete, where residual color vision remains. Complete Achromatopsia is either Cerebral Achromatopsia or Rod Monochromacy. Atypical Incomplete Achromatopsia is a former term describing Blue Cone Monochromacy.
An archaic use of the term Achromatopsia originates from before any understanding of photoreceptors or the mechanisms of color vision. Before the likes of Goethe and Dalton, Achromatopsia would simply describe any total colorblindness, which would include Rod Monochromacy, Cone Monochromacy and Color Agnosia (Cerebral Achromatopsia). Dyschromatopsia was then used to describe CVD where the subject can still see color, but not typically, mostly describing Protan/Deutan/Tritan deficiencies.