Persistence of Vision

"Persistence of Vision" refers to the phenomenon where the retina retains an image for a brief split-second after the image was actually seen, and lends itself to animation by fostering the illusion of motion when we view images in closely-timed sequence to one another. We don't notice the fractional skips between images because that persistence fills in the momentary gap to make the motion seem seamless."

- http://animation.about.com/od/glossaryofterms/g/pervision_def.htm