Weber's law

Psychological concept applied to consumers' purchasing patterns that holds that consumers are more likely to purchase products based on the perceived differences between products than they are to purchase based on the attributes of one product or another.

For more information on Weber's law, visit Britannica.com.

The noun has one meaning:

Meaning #1: (psychophysics) the concept that a just-noticeable difference in a stimulus is proportional to the magnitude of the original stimulus