Definition - What does System Unit mean? A system unit is the part of a computer that houses the primary devices that perform operations and produce results for complex calculations. It includes the motherboard, CPU, RAM and other components, as well as the case in which these devices are housed. This unit performs the majority of the functions that a computer is required to do.
size constancy - the tendency to perceive the veridical size of a familiar object despite differences in their distance (and consequent differences in the size of the pattern projected on the retina of the eye)
Size constancy is an aspect of perceptual constancy in which an object will appear to be the same size to an observer despite of changes in light , rotation, distance, perspective etc. This skill is so important that is appears in an infant only a couple of weeks old. Ebbinghaus illusion.