The Hermann Grid Illusion is an optical illusion that was first described by Ludimar Hermann in 1870. It consists of a grid of white lines on a black background, with a gray spot in the center. When viewed from a distance, the gray spot appears to flicker or move
From Wikipedia
A grid illusion is any kind of grid that deceives a person's vision. The two most common types of grid illusions are the Hermann grid illusion and the scintillating grid illusion.