Many lines of evidence have revealed limitations of the human visual system, most notably that only a portion of available information is processed in a single glance. A degree of conceptual confusion has emerged, however, regarding the underlying concepts or mechanisms explaining limited visual processing in reading; perceptual span, visual span, or visual attention span. While the original definitions of these three concepts are clear and well-differentiated, they are routinely used inconsistently in the literature. The primary goal of the present review is to respecify these three concepts in terms of what they correspond to and how they are determined. Further, we investigate whether these three spans refer to vision-related measurements and/or are based on general cognitive abilities. This review should lead to a better understanding of the involvement of visual functions in reading performance.