Parallax

Parallax is a distorted view of what one sees. For instance, the crosshairs in a telescopic sight. It happens rarely, but when there is space between the crosshairs in the sight, your view changes on the target every time you look. Because of that space, the bulls-eye will appear in a different position as you move your eye. In a strange way, its the difference between what two people see, for they have a separate view.