Just Noticeable Difference
The Just Noticeable Difference (JND), also known as the difference threshold, is the minimum difference in stimulation that a person can detect 50 percent of the time. For example, let's say I asked you to put your hand out and in it I placed a pile of sand. Then, I add tiny amounts of sand to your hand and ask you to tell me when you notice any change in the overall weight. As soon as you can detect any change in the weight, that difference between the weight of the sand before I added that last bit of sand and the amount of sand after I added it, is the just noticeable difference.