Visual angle
The visual angle is the angle under which a visual stimulus appears to the eye. Simply speaking, it is the angle between the light rays from the two ends of the viewed object as they hit the eye.
If one looks at a one-centimeter object at a distance of one meter and a two-centimeter object at a distance of two meters, both have the same size on the retina. Therefore, in many cases it does not make sense to refer to the absolute size of a visual stimulus, if one is interested in the performace of the eye or the first processing steps in the visual cortex. What matters is the visual angle.