Just noticeable difference is an element of psychophysics that is described as being the tiniest change in the intensity of a stimulus that we can discern. Examples of these stimuli include brightness of light, weight, or loudness of sound. For example, when comparing something such as the weight between two objects, the absolute minimum weight difference between the two objects so that it is noticeable is the JND. Another component of JND is Weber's Law, which states that the original stimulus intensity and the JND are directly related. For example, if it were hot outside, we wouldn't notice if the temperature changed degree or two, but we would probably notice if it suddenly got 10 degrees colder. In basic terms, Weber's law means that if there is a strong original stimulus, you'll need a much bigger change for the difference in intensity to be noticeable.
Now-a-days, many companies and professions use the concept of JND to their advantage. People such as magicians use stimuli that fall below the JND to seamlessly make something disappear. Businesses use marketing techniques such using a high JND to make their product stand out. Hey, maybe this is even the secret behind Lady Gaga's success. There have been oddball performers before her, but when she arrived she was so sufficiently different that she got instantly noticed.
One personal experience I had with the concept of JND was when I went to a restaurant called the "Safe House" in Milwaukee. It is a very cool place; you need a password to get in, and once you're inside the walls and pictures move like you're in a spy movie. When I was there, I was sitting at a table with barstools. Little did I know that every few minutes the barstool was sinking a few inches or so. Before I knew it, I was practically sitting on the floor. The rate at which the barstool dropped was below my JND range, so I didn't even detect it.
Check out an example of an artist using the principles of JND for a new interactive experience: http://mediartchina.org/exhibitions/sensorium-of-the-extraordinary/just-noticeable-difference-jnd-ca-usa
And some common JND examples in the media: http://www.psfk.com/2009/03/media-arts-mondays-just-noticeable-difference.html