The Just Noticeable Difference (JND) refers to the smallest detectable difference between two stimuli that a person can perceive. In marketing, it is important to determine the JND for products so that any negative changes are not noticeable to customers, while improvements are apparent. The concept of the JND can also be applied to pricing, promotion, packaging, and product modifications - any changes should be below the JND threshold so customers accept them gradually without discomfort to the perceived change.