The "just noticeable difference" (JND) refers to the smallest change in a stimulus that can be detected by a person. This concept is important in fields like psychology and sensory perception, as it helps to understand how we perceive changes in our environment. For example, if you are holding a weight, the JND is the minimum amount of weight added that you can feel. JND varies depending on the type of stimulus and the initial intensity. According to Ernst Heinrich Weber, a German psychologist, the JND is proportional to the original stimulus level, which is known as Weber's Law. This means that larger stimuli require a greater change for the difference to be noticed.