This article provides a terminological framework to show the relationships among different types of visual deficits. It distinguishes between visual functions, which describe how the eye and the lower visual system function, and functional vision, which describes how a person functions. When visual functions are disturbed, the term "visual impairment" is used, whereas when functional vision is disturbed, the term "visual dysfunction" is used. On the impairment side, the author distinguishes between ocular and cerebral impairments. On the dysfunction side, he uses the term "cognitive visual dysfunctions" to emphasize the different nature of these dysfunctions.