In the anime, A Certain Magical Index, it's discussed in an early episode, and touched on throughout not only both seasons of the show, but also it's spinoff, A Certain Scientific Rail-gun. Among other things, it refers to a person's sense of reality. For example, and this is one of his studies, do people see the same color the same? Use the primary color wheel, Red, Blue, and Yellow. How you perceive the color red, I may see it as your color blue, but because my perception tells me it's red, I'll call it red. Confused? It gets trickier as you approach a genius level, because perceptions are different.
